I guess factors are how much you’re spending, how much you’ve spent before and are likely to spend in the future, how much of a faff the job is, how bored/busy they are, whether they want their lunch, whether the guy before you spent £££s, etc.
I’d be disappointed if, say, I was buying some £300+ forks and was charged to have them fitted but away from the extremes they’re basically doing you a favour and you can’t really complain if you don’t get it.