US taxes - what methods are you using to reliably fetch tax data?

I’ve got an app where taxes need to be applied for a specific state in the US.

Currently I am using the official states tax departments provided tax api to fetch the taxes. This seemed to be working well in combination with using googlemaps api to fetch the complete address for the tax calculation within the state. However, upon closer debugging I realized that the googlemaps api doesn’t always return a full address (the state tax api requires a 9 digit zip to fetch the tax).

So several addresses, maybe 5% or so, errored on fetching the tax because googlemaps didn’t return a 9 digit zip for the tax api to use to fetch the tax rate.

I’ve looked up for alternative methods to fetch 9 digit zip and also reached out to the tax department to see if their api will take full address instead since their online tool uses full address - the api does not have this function.

The alternative methods for 9 digit zip are very limited.
UPS - free but boots me off the api after a period of time if I don’t use the “send parcel” endpoint
a paid 9 digit zip fetching api that is $20 a month just to fetch the zip
googlemaps api which is inconsistent

Previously I was using something like turbotax api but the bookkeeper found that the taxes were not reliably matching the official states tax rates in about 50% of cases. This was a paid api and it was wrong half the time! I tested 5 other top tax apis and also found inconsistencies with them and the official rates which is why we use the official tax api now.

What methods are you using to fetch US taxes and/or 9 digit zips?

Not sure what you use for a processor, but Stripe has this built in. Pretty sure PayPal does as well. Have you thought about those first?