Interstate Sales Tax Compliance: What Accounting Firms Need to Know

Interstate sales tax compliance has become harder for growing businesses to manage. Ecommerce, marketplaces, direct sales, and multiple payment channels can expose companies to different nexus thresholds, taxability rules, filing requirements, and registration obligations across states.

For accounting firms, this creates both a compliance challenge and an advisory opportunity. Clients need help understanding where obligations may exist, which sales channels matter, and when technology or specialist support should enter the picture.

To explore this, Ace Cloud Hosting spoke with Sona Akmakjian, CPA, Global Head of Accounting Strategy & Partnerships at Numeral.

With more than 25 years of experience across public accounting, state tax administration, tax technology, and accounting firm partnerships, Sona brings a practical perspective on how firms can manage increasingly complex sales tax requirements.

In this conversation, she explains how firms can assess economic nexus, avoid common multi-state compliance mistakes, use automation without giving up professional oversight, and decide when sales tax work should stay internal, move to a technology provider, or involve a specialist.

Q1. Why has interstate sales tax compliance become more complex for businesses, especially those selling through ecommerce, marketplaces, or multiple channels?

The 2018 South Dakota v. Wayfair decision changed the entire foundation of sales tax. Before that, a business primarily had to worry about states where it had a physical presence — an office, a warehouse, an employee. Today, simply generating enough revenue or transactions in a state can create an obligation to register and collect there, even with zero physical footprint.

That single shift turned sales tax from a handful-of-states problem into a fifty-state problem for almost any growing business.

Layer on top of that the reality of modern selling: a company might run its own Shopify store, sell through Amazon and Walmart Marketplace, invoice wholesale customers directly, and process payments through Stripe or NetSuite. Each channel has its own transaction data, marketplace facilitator rules, product taxability considerations, and filing requirements. Thresholds, tax rates, exemption rules, and even what counts as a sale for nexus purposes vary by state and change frequently.

Without a system that consolidates and monitors all of that activity, businesses can lose visibility into where they’ve crossed a threshold until a notice — or an audit — tells them. This is where technology platforms such as Numeral can play an important role. Numeral helps businesses identify where they may have registration obligations, monitor economic nexus thresholds, and manage sales tax compliance across their sales channels — giving both businesses and their accountants greater visibility and control.

Q2. How should accounting firms help clients determine where they have economic nexus and when registration and collection obligations begin?

The process should begin with a comprehensive nexus assessment — not simply a review of total revenue. Firms should gather sales by state, transaction date, sales channel, customer type, and product or service category. Marketplace and direct sales should be identified separately, along with any physical activities such as employees, inventory, contractors, offices, or trade-show participation.

For clients using sales tax compliance systems such as Numeral, accounting firms should evaluate how the platform connects to the client’s ecommerce stores, marketplaces, payment systems, and accounting software. Technology can help centralize transaction data, monitor state thresholds, identify potential nexus, and support registration and filing workflows. However, firms should confirm that all relevant channels are connected, data is mapped correctly, and marketplace transactions are treated according to each state’s rules.

This is an area where technology can provide significant leverage for accounting firms. Rather than manually reviewing spreadsheets every month, firms can use platforms such as Numeral to continuously monitor client activity and surface potential nexus obligations. The accountant remains responsible for interpreting the results and advising the client, while technology provides the underlying visibility and automation.

Q3. What are the most common mistakes businesses make when registering, calculating, collecting, and filing sales tax across multiple states?

A few patterns show up constantly. The first is not doing a comprehensive nexus study and registering too early or in the wrong order. Registering in a state before nexus is actually triggered can create a filing obligation before it’s needed. The reverse mistake — registering too late — creates back-tax exposure, penalties, and interest that can dwarf the tax itself.

The second is treating tax rates as static. Rates and rules change frequently at the state, county, and city level, and product taxability isn’t universal. A product taxed as tangible personal property in one state may be exempt or taxed differently in another. Clothing, digital goods, and SaaS are classic examples. Businesses using a single blended rate or an outdated rate table routinely over- or under-collect.

The third is poor exemption certificate management. Missing, expired, or invalid certificates are among the most common audit findings and can turn an exempt sale into a fully taxable, uncollected liability.

Automation can help address many of these issues by continuously monitoring changes, applying the appropriate tax rules, and creating a more consistent compliance process. The key is combining that automation with appropriate professional oversight.

Q4. How can accounting firms use tax technology and automation to manage multi-state compliance efficiently without losing visibility or professional oversight?

Sales tax has reached a volume and complexity level where spreadsheets and manual monitoring genuinely can’t keep pace. There are thousands of taxing jurisdictions in the U.S. alone, and rules change frequently. Automation platforms such as Numeral can handle the repetitive, data-heavy parts of the process — monitoring nexus thresholds across sales channels, calculating tax, preparing and filing returns, and helping manage the compliance workflow.

For accounting firms, the bigger opportunity is what this automation makes possible. Instead of spending significant amounts of staff time collecting transaction reports, reconciling data, monitoring thresholds, and preparing routine filings, firms can use technology to streamline the compliance work and redirect their professionals toward higher-value advisory services.

What shouldn’t be automated away is judgment. Technology does not replace the accountant. Firms still need to decide how to treat a genuinely ambiguous product classification, how to handle a voluntary disclosure agreement for past exposure, or how to advise a client evaluating restructuring.

The right approach is technology that handles the mechanical, error-prone work so the CPA’s time goes toward advising the client, not chasing down transaction reports from five different platforms. Firms should look for tools that integrate with their existing technology stack and provide a clear audit trail. That visibility is important for both the firm and the client if a state ever asks questions.

For accounting firms in particular, this creates an opportunity to scale their sales tax capabilities without having to scale headcount at the same rate. Technology such as Numeral can serve as the compliance engine while the accounting firm remains the trusted advisor and strategic partner to the client.

Q5. When should a firm manage interstate sales tax compliance internally, partner with a technology provider, or refer the work to a specialist?

It comes down to client complexity and the firm’s own capacity to stay current. A firm with a handful of clients selling in one or two states, with straightforward product taxability, can often manage compliance internally with good software support.

Once a client sells in multiple states, across multiple channels, or has products with complex taxability — such as software, digital goods, food, or apparel — the compliance burden can quickly become disproportionate to the fee. Registrations, varying filing frequencies, rate changes, notices, and ongoing monitoring can consume significant staff time. That’s often the signal to introduce a technology solution like Numeral that automates the end-to-end compliance process while requiring minimal ongoing effort from the business and the accounting firm.

This model allows the accountant to spend more time in an advisory capacity rather than performing repetitive compliance tasks. Numeral is particularly well suited to this model because it can provide the technology infrastructure for ongoing sales tax compliance while allowing the accounting firm to maintain the client relationship and provide professional guidance.

Referring out to a specialist firm, an internal SALT practice, or a consultant makes sense when the situation goes beyond ongoing compliance: significant historical exposure requiring a voluntary disclosure agreement, an active multi-state audit, complex nexus questions tied to M&A or restructuring, or industries with unusual sourcing and taxability rules.

Ultimately, the best model isn’t about replacing the accountant with technology. It’s about using technology to make the accountant more valuable. When routine compliance is automated, firms can serve more clients, expand their sales tax capabilities, and devote more time to the advisory work where their expertise has the greatest impact.

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Sales Tax Compliance Is Becoming a Systems Problem

Sona’s perspective makes one point clear: interstate sales tax compliance is difficult to manage as a collection of isolated filing tasks.

As interstate sales tax compliance becomes more complex, accounting firms need the right combination of technology, professional judgment, and a reliable IT environment to manage it effectively.

Platforms such as Numeral can automate the compliance workload, while Ace Cloud Hosting can help firms securely host and access the accounting, tax, and business applications that support their wider client workflows.

Together, the goal is not to replace the accountant, but to reduce operational complexity so firms can spend more time advising clients, managing risk, and delivering higher-value services.

Navigating Interstate Sales Tax Compliance: What Accounting Firms Need to Know