You should try this website - www.payloadz.com
They are a reputable established service that is free to sign up. Basically, it does exactly what you described as needing: Put a button on your site, after clicking they use paypal to buy it, and then it provides the link to download your file, as well as emailing a copy of the download link to the customer.
Only thing about it is some people may not have heard of this company and would be suspicious of purchasing from it, so you might need to put a little disclaimer explaining something about this site being a valid external vendor with a link for them to investigate further if they wish to.
Depending on the type of file you are wanting to sell, you may want to also check out www.lulu.com, but they tailor more towards e-books and stuff, but its the same concept, and they are a little more well known in the web community, in regards to catering to the more suspicious customers.
Hope you find this useful still..
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