Ok, if you want instant digital delivery, I will try to go into this in osme depth. I presume you accept Paypal as payment.
Link hidden: Login to view is a FREE HTML editor so you can see what your creating and its dead easy to use. Once oyu have your page ready go to paypal and create a buy it now button, (tutorials available on Paypal)at the bottom of the create button page there is an option for 'more information', click here and there is a return URL that Paypal will send once the payment has been approved, this is where you put your download URL or thank you page - but bury it DEEP in your website otherwise people will find it and get it free. When I say bury it, do something like this:
this will be the thank you page, then on this page you put CLICK HERE and hyperlink it to the actual download without giving away its location!
It should work like this - buyer clicks on buy now button - Pays - then is redirected to your thank youpage - gets download - DONE.