This could be solved with a simple <link> standard.

If we're talking "direct" payments, as much as it pains me to say it, it would be bitcoin, wouldn't it? Just put a wallet address in a <meta> and you're done. But with its operational overhead it may be a waste for micro-transactions compared to traditional payment methods.

To be honest, the real problem isn't that there is no way to mark this up, is that even if there WAS a way to mark this up, most people wouldn't because it's not a browser API. People don't even use RSS because browsers don't tell users about RSS links. There needs to be a tangible consequence of using a tag for people to bother with it. If a $ doesn't appear on the address bar, what is the point?