Upload my software from my web site, after customer pays?

I’ve been searching for some time for a means of allowing my web site customers the option of down loading the software I sell at the site, rather than waiting for a disc to arrive in the mail. I’ve purchased software my self this way, and it looks pretty basic. I guess the way it would work would be for the customer to click the “add to cart” button, where he would submit his credit card info. Then he would be sent to a private page or something where he would down load the software. Can anyone help me with this?

Thank you,
Bob K.