Yes, you can complain.
Don't forget if it is a delivery issue, talk to the delivery supervisor.
IF you go only to the postmaster, that postmaster may or may not give the complaint to the delivery supervisor.
Basically the chain goes: (i may be off a bit, this is old info)
mail carrier
Route supervisor
Postmaster
Some places, (in my largish city), the postmaster is not at my particular post office. THey are in the larger main office, where I go to a small regular post office.
Because of this I had to talk to a delivery supervisor.
Also they do let people know who complains. I wound up complaining about a counter clerk (She claims she can't lift over 5 lbs.. hello, the PO requirement is 75 lbs! If you can't do your job, QUIT!) I had her refuse me three times, and each time the supervisor had to come and talk to her. Now whenever I am in there, with my multiple heavy ebay packages (pc's and laptops), she convienently closes her window.