So what happens when everyone adopts slightly incompatible variants of this system?

Endless loops of "Please pay me 2 cents via PayPal so I will read your message with the Subject: Please pay me 3 cents via PayPal"

Oh, you'll put in an exception for those messages?

Next message is from your mother. You're going to charge her 2 cents?

Ah, whitelists. Management problem.

Also: PayPal can't efficiently handle micropayments. (Well, for 33 cents apiece, they will.) So you need your own micropayment system. Which will be incompatible with your prospective customer's micropayment...