"Please, enter your PIN" (for card payments) - Proszę wpisać kod PIN

"Have a nice day" - Miłego dnia!

We'd actualy normally just say the numbers when sking for the amount of money to be paid. You can add "proszę" at the end of the sentence, and that's about it

as for "do widzenia", well, I do think it's the best solution. We don't really use any other parting phrases in stores. I suppose you could say "cześć!" or "na razie!" if you insist on making it less formal, but I'd find it rather weird. You could probably get away with it, given the fact you're a foreigner, and people would appreciate the effort either way, but it still sounds awkward, especialy if you'd use it towards an ederly person.

I think it's very nice of you to do that. I remember I couldn't hep but smile every time people tried to speak Polish to me when I was living in the UK, made me fee just a little more welcome