So I feel pretty certain I'm 'use'ing Registration correctly, since it works fine for the renew path.

Don't feel, be. You have a line number from your error message; insert require Registration; immediately before it and see what happens.

The error literally means that upon attempting to execute that line, the interpreter could not locate a code reference for "testMethod" in the symbol table for package "Registration". Unless one of your packages is mucking around in symbol table, it means that either Registration doesn't contain a subroutine definition for testMethod or you never compiled Registration. As your other script functions as expected, the first option is off the table.

If the above does not resolve your issue, we need you to follow chromatic's advice, and post a complete code snippet that replicates your issue. I have frequently found constructing such a snippet is a great way to track down the problem.