We don't have a website so all transactions are entered in our UI using customized transaction forms. The customer center is a custom customer center role that we created.

It sounds like a permissions problem. You might want to see if that custom role has "Transactions > Find Transaction" and "Transactions > Sales Order" permissions. (It sounds like it needs "Sales Order" but that's a hunch.)

Hmm..."Find Transactions" is set to "Full" and and "Sales Order is set to "Edit". Are those correct?

Yeah, it sounds like it.

Thanks. If anyone else has any input on it, we'd greatly appreciate it!