$75 for was attaching your transmitters and any hoses that needed to be attached (this is literally as easy as screwing in a light bulb) and pressurizing the regs to make sure there are no leaks, and checking to see if IP and cracking effort are within specs. 5 minutes would be a long time to do this for each regulator set. If they have more advanced testing machines (unlikely) maybe they could do some flow-testing, but that is so not necessary for a new regulator.
I don't see what the problem is here. It wasn't just about attaching the transmitter; there were other parts he bought elsewhere. The LDS's responsibility is only for the parts he purchased from them, not what he bought somewhere else. Not only they are going to attach all parts, but we also test everything to ensure that cracking pressure, IP, etc., are within specs. That's fair.
An LDS isn't a charity, and they should charge for their time, including overhead, for the things the customer didn't buy from them. There's nothing more annoying than a customer coming in with a bunch of unconnected hoses they bought cheaply elsewhere and expecting the LDS to put everything together, test it to make sure everything is within spec, and not charge for their time and effort.
During the time I owned a dive center, when a client buys a regulator package from my LDS, I connect everything and check it as if it’s in for the annual check. I do the pressure checks, cracking pressure, venturi, etc., all at no charge.
If somebody brings in equipment they bought elsewhere and wants me to put it together, I'll do that no problem, but I will charge them.
Our service department rates, including the cost to put things together, are listed on the "Price List" that is prominently displayed on the wall in the store.