When the source code will be published?
May be Im wrong, but according to the interface (ABI) the sender does not know how many tokens he transfers to the receiver.
How can the payer check that the secret code is for particular amount of tokens?
When forming an invoice for payment, the recipient of tokens indicates their number and transfers the goods (payment code, coordinates, download link). The buyer conducts the transaction and after that gets access to the purchase.
Oh, the buyer could check what he pays using showBillInfo function. Right?