1. Oops. I forgot to say that this was only to apply for sales taxes on out-of-state sellers.

2. I did remember to mention that for #2.

3. The rates that apply to a particular location in a state might change infrequently, but there are a lot of locations, so even if each location's rates change infrequently there can still be a lot of rate changes within the state each quarter.

For example, in my state (Washington), there are ~1 million 9 digit zip codes listed in the rate data the state makes available. Here is how many of those areas had rate changes compared to the previous quarter starting from 2020:

2020q1 14930 changed
2020q2 121464 changed
2020q3 9084 changed
2020q4 1779 changed
2021q1 185365 changed
2021q2 115306 changed

(This is only counting cases where the zip exists in the rate tables for two consecutive quarters. There are also zips being added and removed which I'm ignoring).

In those 6 quarters many places actually changed more than once, the most extreme being 98520-6503 which changed every quarter.

2019q4 8.98%
2020q1 8.80%
2020q2 8.90%
2020q3 9.08%
2020q4 8.90%
2021q1 9.08%
2021q2 8.90%

Among all the places whose rate changed sometime in 2020 or 2021 (so far):

436523 changed 1 time
4533 changed 2 times
654 changed 3 times
84 changed 4 times
7 changed 5 times
1 changed 6 times

4. SST would be great if every state did it, but as you note it is voluntary, and as you noted in other comments I believe several states apparently have no interest in joining. If the Feds did it, they could make it mandatory, and they could make it simpler (such as making it purely zip code base5 (hopefully 5 digit only) instead of address based).

With it zip based and updated at most once per quarter, almost all the complexity of dealing with finding rates go away. Heck, I would probably remove the address entry fields from our shopping cart because we sell downloadable goods and tech support for those goods. The only reason we have to ask for address is for sales tax lookup.

What I'm basically suggesting is something similar to the EU's VAT MOSS system. Here's how we deal with VAT for our EU sales. (1) It is one rate per country. I had a local DB with the rates. (2) Once a quarter I make a CSV file that lists country, our sales in that country, the VAT rate of that country, and how much tax we collected. (3) Once a quarter someone in ops uploads that CSV to the Irish tax authorities and we pay them the total of all EU VAT we owe for that quarter. (4) The Irish tax authorities forward the data to the other EU countries along with the tax owed (logically...I'd assume that physically they do something like net out things first. Just as we pay our Italian and German VAT through Ireland, there are others paying their Irish VAT through Germany or Italy, and there is no need to have say Ireland pay Germany at the same time Germany pays Ireland).

I don't see why the states could not do something similar, so that I just send a CSV of state, zip, sales, rate, amount collected to one state along with the total tax collected and let them distribute it. Maybe have the Feds set up a clearinghouse for the states to use that actually deals with netting things out and settling between the states. Might as well then go one step further hand have the sellers just upload directly to the clearinghouse.