"Ga i baned o goffi os gwelwch chi'n dda?"
Translation:May I have a cup of coffee please?
Yes, it is the usual expression for 'please', very similar to the French s'il vous plaît.
I think the question may have been about using chi vs. leaving it out.
At the top of the page, I see chi'n, TracyMarti8 asked about yn.
Both versions are common, aren't they?
We stick with just 'please' because, with rare exceptions, it is what people actually say.