Thanks for the links. I can see my two blogs with domain mapping upgrades in: https://dashboard.wordpress.com/wp-admin/index.php?page=my-blogs&show=hidden
Just no option to "Delete Payment Data".
The only Admins on these two blogs are myself (the username I am currently logged in as and commenting on this forum) and the client.
All I can think of is that I set the blogs up several years ago using a different WordPress.com username, swapped the Admin role to another of my usernames, and I have since forgotten the original username.
It seems odd that domain mapping upgrades for the two blogs are showing "Active, Manual renewal" yet they both automatically renewed and billed my credit card this year.
I appreciate your help and links but I'm really stuck with this now.