The Order Page is often combined with the Sales Page, as in our example.
Some people don't like to tell the reader the price on the Sales Page (a tactic I really dislike because if they're afraid to let me know the cost up-front it's probably overpriced anyway).
Whether part of the Sales Page or not, the Order Page (or Order Link) is basically just a BUY NOW button. I would probably avoid ADD TO CART buttons in these circumstances and save that approach only when you have several products presented in a catalog format. Here are two BUY NOW buttons, feel free to use them. PayPal also offers a selection of BUY NOW buttons.
Most people probably use PayPal as their payment processor, so that's what we'll
go with here. PayPal is free - BUT you want to set yourself up as a Business
Account so you can use their "Seal of Verification". They will ask for
your bank account information - account number and routing number, then make two
deposits (under a dollar) into your account. You then tell them the amounts of
those deposits. This confirms to them that you actually do have a working bank
account ... and it gets you listed as a Verified PayPal member.
Personally, I think it's a good idea to explain PayPal briefly to the buyer (do this on a linked separate page). Online buying is soaring, but you don't want to forget the newcomers who may be a little leery and don't quite understand. PayPal does not guarantee downloadable products. I don't know that I would go out of my way to point this out, but I would make darned sure each of my own products has a good guarantee and make darned sure I honor it.