Jeff,
I'm sorry, but there is no feature in our system to delay when the buyer will gain access to the Thank You/Download page. As soon as the buyer is done checking out, PayPal will provide them a link to that page. If you could change settings in PayPal so they did not provide that link, then buyers would only be able to gain access to the Thank You/Download page via a link in the Thank You E-mail we send.
So you know, this could cause unforeseen problems, if for example a buyers Spam/Junk filter does not allow the e-mail from our system through.