Yes, I agree you would have to integrate which you would not have to do if you used only one channel (e.g. eBay). But other tax service providers could ease the integration by allowing bulk import (provided each channel let you bulk export the tax data). However, there's no incentive for eBay or Amazon to make bulk export easy. So realistically you would ignore all of Amazon and eBay's sales tax calculations and do your own with a reliable service for all channels. But I suppose Amazon could throw a wrench into that by adding sales tax themselves whether you ask them to or not.
I appreciate your insighta. I assumed it wasn't easy to compete with Amazon especially if Amazon is one of your channeks and they monkey with those sales. But again I think there are potential third party service solutions to some of this. But as STC seems to demonstrate, keeping up with the changes may be difficult.
In this example, it's not the question of the rate - it's the question of which county gets credit for the 2% charged above the 4.75% state rate. I have to report (monthly) which tax due by county. The state takes care of distributing to the counties (and/or cities). If I used STC, it would all go to Forsythe County - even if the shipment went to Davidson. If I were audited, I'd get the fine or penalty.
And yes, I have a love/hate relationship with Amazon (love to hate them :-> ). I get a lot of orders through them as a 3rd-party seller in their marketplace. But they also compete directly against me on certain listings, and where they're not authorized by the manufacturer of the goods as a dealer. The manufacturers won't take them on - either because they like the revenue too, or because they're afraid to take them on.