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Polish post office vocabulary [8]
Usually in such situation in foreign country it's enough to hand addressed letters to the person in the till. They will choose and put the stamps for you and tell you how much to pay. You will most likely be asked if you want "priorytet" it's a fast letter service, the difference in price is very small, but the letters and parcels usually go much quicker, so everybody I know usually take this. Air mail is weighted.
It's harder to buy stuff like stamps, hmm, I don't know how good is your Polish, if it's very bad or non existent, maybe it would be better to have those things written on a piece of paper or just look for an English speaking cashier. Or just buy a sheet of stamps and ask how much it is for a letter to certain place (I think you can also check it online). Stamps for domestic and international letters are the same, just sending abroad is more expensive. I'd say check how much you pay for a letter abroad and take it into account when buying stamps.
Anyway:
stamp - znaczek
letter - list
parcel - paczka
signed for letter - polecony
priority (faster) - priorytet
I'd like to buy stamps for domestic letters - Chciałbym kupić znaczki na listy krajowe.
How much it is for a letter to ... ? - Ile kosztuje wysłanie listu do ...?
I'd like to buy some stamps. - Chciałbym kupić trochę znaczków.