Hi, @s.bingham.miller
I'm sorry to say, but Proton Mail domains are not currently allowed to process renewals due to the high level of potential fraud.
You may use an alternate email address as the login, and then your purchase should proceed. Alternatively, you can make a different Family member the administrator if they use an email with any different domain.
OK, I switched to a different email, tried to renew, and again got a failed payment message.