Learn Polish: Tak, proszę paragon. - Yes, a receipt, please.

It is both! Proszę literally translates to the verb "I ask". But in everyday Polish, it is the standard way to say "please". When you pair proszę directly with a noun like paragon, it translates perfectly to "A receipt, please" or "I would like a receipt."

Why doesn't the word paragon change its ending here?

Because proszę is a verb meaning "I ask for", the thing you are asking for takes the accusative case (it is the direct object). However, paragon is a masculine inanimate noun (an object, not a person or animal). For these types of words, the accusative form looks exactly the same as the basic dictionary form!

Could I also add the word o and say: Tak, proszę o paragon?

Yes, you can! Proszę o paragon is also very common and means the exact same thing. In fast, everyday shopping situations, dropping the o is perfectly natural and polite.

What is the difference between paragon and rachunek?

A paragon is a standard printed cash register receipt that you get at a supermarket or shop. A rachunek is a bill (like at a restaurant) or a more general receipt. Since you are learning about prices and shopping, paragon is the exact word you will hear when the cashier rings up your items!