Well, no one wants to pay but some people don't want to feel guilty about not paying. In my last job, we were told to say, 'How would you like to pay?'. I quickly got bored of the resulting sarcasm from every other customer.
Don't take my word for it but i imagine it's formal and colloquial. Although it could be a North/ South thing, im from the south and only tend to hear os gwelwch chi'n dda.
Glad this is not Plîs as it's informal. My experience is not that likely to use Plîs to ask the assistant for the bill (unless it's your mam's place :) )