You would need to install a 3rd party Shop system, which automatically processes the order/email once paypal API has informed it that payment has been sent. Have a look through hotscripts.com
You don't even need to know the paypal api system. You can just use the thankyou page part to send them to your script with a key, if they key is right let's them download it. I've made quite a few basic programs like that before, it's not hard to pick up.
^what if the client refreshes the thank you page? Wouldn't your site think they made 2 purchases instead of 1?
I use Payloadz, which costs like $5 per month and it allows me to create a PayPal button for the things I sell and then sends the files to the buyer as soon as the payment goes through. I haven't had any complaints on it yet.
What I have used in the past was a paypal merchant link linking to a file download from rapidshare or some other form of that. Of course, you need a merchant account first. They can be taken to the download page AFTER they have purchased a product, or you can send them to another site if they do not want to purchase it.
I guess its only been done, if you have a merchant account and that its been already verified... BTW whatz your paypal monthly withdrawal limit ?
the thank you page is an easy way to get it done. also the api is a way to get the job done. if you want to be sure with the thank you page, why dont you send out mails automatically to all email adresses that pay via paypal? if not using outlook what makes it possible, there are some neat progz that do exactly this.
if you are using WP then you can easily find some plugin...for websites also there are many software which can be used as shopping cart purpose.