Honestly, I would just sell off my own site if processing payments were simple to add. (And I suspect they are easy to add but that I'm just lazy and wouldn't know which service to wire up in the HTML.)
Since my scale is small enough, even if the user forgot to enter the order ID, it'd still work because I could just map the order and payment manually by timing.
If you don't want to create your own order handling website, even a Google Form would work for that. Just get the user to pay you on their own and fill in a form. Then you'd do the payment mapping manually and send them the product.