Actually, the federal government should oblige each member state to provide the algorithm, and sign it cryptographically and have it expire every X fixed time interval, and have signed algorithms for the current and next time interval, so that software can automatically fetch and stay up to date.
Then the "business opportunity" of navigating FUD evaporates. Currently any such enterprise charging for such a service can spend a fraction of their budget lobbying against harmonization...
Since it would be an obligation of the states to the federal government, these algorithms (provided by each member state) should be hosted on a fixed federal government site.
Time to start a petition?
What is the most convenient format for this layered geographic data? Are the tax district boundary polygons already otherwise available as open data?
What do localities call these? Sales tax tables, sales tax database, machine-readable flat files in an open format with a common schema?
How much tax revenue should it cost to provide such a service on a national level?
States, Counties, Cities, 'Tax Zones'(?) could be required to host tax.state.us.gov or similar with something like Project Open Data JSONLD /data.json that could be aggregated and shared by a server with a URL registry, a task queue service, and a CDN service.
While the Bitcoin tax payments bill passed the Senate and House in Arizona, it was vetoed in May 2018. Seminole County in Florida now allows tax payment with crytocurrencies such as Bitcoin:
> According to a press release, the county will begin accepting Bitcoin (BTC) and Bitcoin Cash (BCH) to pay for services, including property taxes, driver license and ID card fees, as well as tags and titles. The Seminole County Tax Collector will reportedly employ blockchain payments company BitPay, which will allow the county to receive settlement the next business day directly to its bank account in US dollars.
This could also help reduce the costs of tax collection and possibly increase the likelihood of compliance with the forthcoming tax bills!