some of the whingers on my round seriously need a taster session with the local "big boy " firm, im forever hearing of how he deals with wasters ,no messing for sure
I dont do a letter as such. i wait until a time the custy is not home and then write on thier bill.... "please note a small price increase from next visit to £?" That way i dont get into customer haggling, awkwardness etc. if they really dont like it im sure they will let you know.