I don't bother with letters on this now, (unless i planned a big increase)
If custy's in and I put a pound more on, i just say new price.
they either say how much is it? (custy doesn't remember, so why draw attention to increase)
Custy says oh has it gone up? (me: yes everyone's gone up £1)
Or if custy's not in just leave bill with new price on.
When i use to announce a price increase it was usually met with moans and groans; and it feels a bit like asking custy's permission if it's alright :
when i goto to petrol station or tescos they don't give announcements telling of price increases