My cleaner has commented on the waterless diffuser in my bathroom, so I've got her one for Christmas with the same oil I put in mine. I will also pay her for her usual day in the week between Christmas and NY, but give her the day off.
I don't tip or buy for people like the binmen or postie, although when my decorator has done a job for me, I get her a gift too. She's done several rooms over the years, so I know her quite well now, and find giving money awkward, plus it's her business and I pay her the rate for the job, so paying more seems pointless. I would rather give a luxury item that I know she wouldn't buy for herself as she has a young family. Fancy bath oils or a good candle - that sort of thing. The value roughly depends on the size of the job, but is not worked out to the penny.
When I had the kitchen done, I noticed that the fitters were always eating sweets, so I got them a tub of Celebrations and gave it to them on the last day. I might have got whisky, but one of them had said he was a recovering alcoholic.
I think a small gift like that is a way to show you are pleased with the job, but is not as awkward as giving money as well as paying for the work.