You can increase the price without notice but they are not obliged to pay it for obvious reasons.
So why not just Inform them surely that's the right thing to do.
Mostly because it feels a bit 'tedious' to explain to a domestic customer that their property is going up by a quid or two.
For commercial, it also feels tedious. One owner of a medium sized business with around 40 employees (that I know socially from the local running club) once said to me that the window cleaning costs are a tiny fraction of their overall outgoings.
Do they really want to be bothered with a notification about a £5 yearly increase?
I just stick 'em up.
I don't explain to any domestic customer nor do I ask. I have all their phone numbers I bulk text them via cleaner planner it takes literally 10 mins to inform the lot.
As for commercial you should probably email or letter them.
I get it's tedious but just because you can't be bothered doesn't make it right. Most won't care but no one is obliged to pay a increase in price without notice unless its contracted so if they turn around and tell you to shove it you can't complain really.