I'd like to provide further clarification, Steve. 

A purchase order is a vendor transaction. Though you've changed the invoice template's title into a Purchase Order, it'll still remain as an invoice. Thus, it'll show up in your sales transaction.

Yes, you can change the invoice title to a purchase order. You'll need to do this in MS Word and then import it into QuickBooks.

Every time you create an invoice and use the template, you can click Customise at the bottom. However, there's no option to do it in the mobile app. This is actually a good feature suggestion that I can send to our engineers. 

Let us know if there's anything else that I can help you with. We got you covered!