At tookapic.com I use Paddle and they simplify taxes a lot for me. However along with that comes a lot of limitations. Their support reply time is also very slow. The API does not offer everything I'd want. For few weeks I used fastspring.com which does pretty much the same, but still - even more limitations in terms of design.

So for simple payment solutions there's Paddle.com and Fastspring.com. If you want something more advanced and flexible, I'm afraid you'd need to pay extra for services like Avalara.