There are plenty of Bitcoin Payment APIs... try searching Google.
Woocommerce seems to be a popular one... and there are bitcoin plugins for it.
Mycelium GEAR is one that allows you to integrate bitcoin payments directly to your own wallet... without a 3rd-party, not sure if it supports handing the digital download thing tho...
You could also try using "Selly" etc.
No i need to add pay with bitcoin bottom ?? like sites that support bitcoins