How to greet the store personel in Polish?

Alright, so I'm walking into a store, and I need an envelope, some lined paper and a pencil or a pen.

How do I greet the store personel in an appropriate way (The difference between a man and a woman?) and how do I ask if they sell any of these articles?

You can say regardless of who is selling:
"Poproszę o kopertę, papier w linię i długopis"

To a female: "Proszę pani, czy ma pani koperty..."
"czy można kupić u pani koperty..."
"czy sprzedaje pani koperty...."

To a male: "Proszę pana, czy ma pan koperty..."
"czy można kupić u pana koperty..."
"czy sprzedaje pan koperty...."

I think this is the most common way of asking for things an the simplest at the same time. My husband always uses it (Polish learner) and so do I (native Pl). You can also omit 'o'.

the most simple

"Panie, daj mnie pan koperty"

I think, the most natural will be: "Dzień dobry,czy ma pani/pan koperty...."or just"Dzień dobry, chciałabym/chciałbym kupić kopertę....." or "Dzień dobry,poproszę kopertę...."You see,there is not a one way:)

I like that one!

Okay, so now I've helped myself to the things I wanted to buy and now I want to pay for these articles ofcourse, but I only have a creditcard.

So how do I ask if they accept credit cards?

czy moge zaplacic karta kredytowa?