One additional last thought:
I shall get to the bottom of my no sales mystery with PayPal, it's not up to you to solve their problems
Your HTML for the Paypal button to buy a product appears correct. As long as you have that HTML on your website, and it does take the potential buyer to Paypal, where they can choose to pay via credit card or Paypal (as I've demonstrated when I tested it out), then I don't think the lack of sales is a problem on Paypal's end. I think you just need to work on your website's marketing to get visitors to buy your product.
So for the meantime, this would be fine, as I have a sale and the buyer has the product, so main goal achieved, it's all a shop wants really.
Not necessarily true. A shop doesn't want the HTML page that includes a link for everyone and anyone to download their paid product to be accessible. I would implement PDT sooner rather than later, so that you can ensure that only people who paid for it can download it!