I prefer to pay in cash when I can do so. I think payment by cash and by barter will be better, in situations where that works.

However, for computer payment, I had another idea is to make a "computer payment file" that contains the order division and payment division, and with encryption and signature, and send that to them. You will first receive the file telling what payments are acceptable and can use that to make the file to send to them. Stallman mentioned the possibility of payment by cash by pay phones (or with a prepaid phone card), so that might be one way to do it, too; after you figure out the price, you can receive the payment code and include that in the payment file. Other methods of payment would be possible (e.g. store credit), so the payment file can work independently of what kind of payment.