I found the best option was a loan unit, this can often be supplied from the shop doing the repair at a small fee + freight.
I once heard a bloke that knew a bloke say that "loan units can often be found on eBay. You install them and they test "fine" and get signed off in the Log Book (not MR)- next attempt for some reason it goes U/S. The faces all look the same and an AWI is not going to remove your radio's to check S/N's - Thats the ATSB's job.
Flying to a facility will cost ferry hrs and down time. Not often they will jump straight onto your com - needs to be left a few days to adjust to the workshop room temperature before it can be touched by the Bench Tech's bare hands. There is only one problem, and that is once the unit is removed the aircraft is grounded until a) the com is replaced, b) An EO is granted, c) A permit is granted by CASA - gave up on that many years ago.