I am the technical contact for a few domains. It would be nice if the billing and/or administrative contacts could log in or call in and pay their bill when they need to. I don't want to give them access to all of the information of all the other domains nor do I want them even knowing which other domains I'm managing.
I often wonder why there's so much secrecy in paying a bill. If I want to pay a bill, even if it is someone else's bill, I should be able to do that with no issue. If I want to pay the renewal for ithinkthisismydomain.com I should be asked if I want to make this payment for ...$$. If yes, the name is noted on the account as the person that paid and that's that. Otherwise a way for them to make payments only (not changes) should be easy.