Hello Katie,
This is a follow-up to the ticket you had submitted regarding needing to specify the template when creating an invoice. It is possible to set the defaults based on the saleable items used in the Invoice. I have added screenshots where you can set the default below. Please note, the default invoice template is set in a different location based on the saleable item used.
Membership Dues Fees- Setup>Membership>Types>Click on the pricing for the membership
System Default Invoice Template- Setup>Finance>General Settings *Note- This will be the predefined invoice template used for all items by default including Event Registrations
You can set specific fees to use a particular invoice- Setup>Services, Products, and Commerce>Fee Items>click on the fee item>check the box for advanced options on the bottom of the screen and set the invoice. If you use multiple items that are set to different invoice templates, I believe the system will use the default in the General Finance Settings. *Note- Membership Dues Type items cannot be set to a specific invoice in this area as the invoice template is set in the membership pricing area.
Thank you and please let me know if I can be of further assistance with this,