Seriously?! Okay, maybe to write the code, but to collect the tax rules and keep them current?! And tax what? Some states tax everything--goods, food, non-food, medicine, services, shipping, etc. Others don't. It is not simple. And the laws change.
And it is not just figuring out the tax rules in around 10,000 jurisdictions, then collecting. Now I have to to file returns. Many states require I register as a foreign corporation to do that. Ah, that is even more complication. All those filings, monthly, quarterly, and annual reports.
Hiring a developer to write code is the easy part. Which is why I don't pay developers much.
Here's a website (I'm sure there are many others) that will tell you how much to collect:
http://www.thestc.com The filing is then up to you. But there are probably other services that will also file for a bigger cut of your pie. But there's always competition in this space because providing such a service has very low costs (just the manpower in keeping it up-to-date).