I've tried something similar with discount setups when running a small online store. I used sequential discount vouchers; keeping track of usage was tricky initially. What worked for me was using codes that expired after a single use, forcing customers to redeem them in order. It made things more straightforward but still required an organized system to manage.

Whoa, those Avatax prices are insane! I use TaxJar, it's not directly integrated with Abantecart, but their API is pretty straightforward to work with (a developer could hook it up relatively easily). There are also some cheaper, simpler tax calculators out there, but you'll have to manually enter the data, definitely more work, but way less expensive than Avatax.