The group tag didn't change anything. (That would have been a really stupid restriction, lol)
But paying only 1 week at a time worked. It didn't let me pay the 4th week so maybe it's really strictly enforcing the 4 week max. (I was originally trying to pay 4 weeks at once.)
Thanks for the suggestion. Even though wrong, it got me thinking on a different line and that allowed me to find the answer.