USA, Canada / Sending Mail from USA to Piekielnik, PL and back [6]

Hello,

I want to send a letter from the USA to a parish in Piekielnik, Poland and have the address given to me by someone in Piekielnik-

Parafia s. Jacobi
Piekielnik 331
34-472 Piekielnik

Is this address correct if I add , PL after Piekielnik?

I know the postage from the USA to Piekielnik, PL; but, how do I include a self-addressed envelope with the correct postage for them to return to me?

Thanks.