I would write the letter if for no other reason than your own integrity and knowing you didn’t sit idly by and do nothing. Will it make a difference or even get read? Probably not, about 12 years ago a customer on a route I was holding down while the regular was out mailed a letter to my postmaster (whom i got along with well) saying how nice, courteous and good I was and how no replacement for the regular would ever leave her not knowing he wasn’t on that day unless she saw me working. She asked me once about it if I ever heard from the pm, and I said no. So being the confrontational irishman I am I questioned him about it and he said something to the effect of once I saw it wasn’t a complaint it went into the trash, my job isn’t to hear or know what you’re doing right but what you’re doing wrong. Basically they’re gonna turn a blind eye to what they want, but like I said at least you’ll know you did nothing.