Cargo has nothing to do with it.

UPS longevity pay seems to work for them and I guess the UPS pilots are OK with it.......but
FWIW, I believe that airplane pay should be broken out by it's ability to produce revenue.

A 737-900 should pay a few dollars more than a 737-500.......yes Pilots can fly both but when they fly a 900 it should pay slightly more than a 500 because of the extra seats.......i.e. revenue.

A-319, 30 and 321 should pay more respectively

B-757-200, 757-300 ditto

B-767-200, B-767-300 and -400 ditto


Same for FedEx
A B-757 should pay more than a B-727
A 310's verses the A300-600. The A300-600 should pay a few dollars more than the A-310.
Same for MD10-10 verses the MD-11.

Cargo vs PAX should make no difference.
Maybe a cargo plane generates more revenue than the same PAX airplane.........but Apples to Apples and Oranges to oranges.......a bigger airplane generates more revenue than a smaller one carrying the same Payload, be it People or Cargo or a mix of both.