There are already a lot of defined (if not quite standardized) possible values for the LINK element in XHTML. We're using them for RSS Auto-Discovery and, of course, for stylesheets already. But what about something like:
<link rel="appendix" type="text/html" title="payment" href="URL of Wishlist or Paypal Page" />
A standardized way of linking to the page where you pay for your patronage of a site. Granted, calling the payment page an "Apppendix" of the site you're visiting is fudging things a bit. Maybe there's a more elegant way?