Allow a Flat-Rate to be ADDED to Project hourly for "Service-Calls"

When service personnel perform "Service-Calls", there is typically a Flat-Rate that is charged "just to show-up onsite" in addition to the hourly rate for perfoming the "service". This is to help cover travel costs & vehicle wear-&-tear. It would be very helpful if we were able to to add BOTH of these fields in the "Project" settings for the invoices instead of only "one or the other" (Flat-Rate AND Hourly, instead of Flat-Rate vs. Hourly)