I think the tax is basically on the profit made when you add up costs and expenses. Say:

Before: Importer pays China $10 for widget, pays $2 duty, sells to shop for $12 - profit zero, tax on that zero.

Now: Paid $10 for widget. Paid $2 duty, sold for $12, $2 refunded - profit $2, pays tax on the $2.

At least that's the normal way of doing accounting. There can be odd exceptions and complications in local laws.

Yes, I think that's the starting point. Another part of my question was whether a CPA applying GAAP would recommend recognizing the $2 as other income, or else as a liability against a future claim from the customer who bought the widget and is now seeking a partial refund.

I did what passes for research these days and concluded that if the claim is "probable and estimable," then it could be recorded as a "contingent liability" rather than other income. Relevant facts would include whether the tariff refund included a pass-through refund mandate (unlikely with this administration), or whether class actions for refunds against merchants were pending (inevitable).