An invoice should be enough
I'd take it one step further,
If the FCC want me to replace a piece of kit and I am saying it will cost too much, I should be able to send the FCC the details (including serial number) of the piece of kit that needs replacing along with the invoice for it's replacement.
If the FCC pay the invoice I get a nice new shiny toy delivered and I just need to schedule replacement of the old with the new. It's up to the FCC to determine if the nice new shiny kit is an appropriate replacement for the old equipment.
With the make, model and serial number of the old equipment the FCC should be able to determine if it's a reasonable replacement and I'm not trying to swap an 8 port 10/100 Netgear switch for a 6U 10GB Cisco switch/router