Follow-up from "Add GitLab credits support to auto-renewal presenter"

The following discussion from !13696 should be addressed:

• @minahilnichols started a discussion: (+3 comments)

  note to reviewer: moved this to base presenter as this was duplicated logic.

We should add tests for when the unit_type is different in these method definitions, with it being invoice_item.unit_type for Saas and user for SM, so these are not missed if changes are made.