Why can't I edit club orders as an Admin when the club package is configured to Set Items?

If you use Set Items, even the Admins cannot override the package content.

The reasoning behind this is if you are allowing members to email or call you and you make the changes, why not allow them to do it online? It seems like this is a User can Choose club.

If you really want to make an exception for a club member, you can skip their club package and create a new Inbound orders with all the extra items. However, you can't change that to a Club channel.