Learn Hardcore Croatian: Za povrat ili zamjenu trebam račun i potvrdu iz trgovine. - For a refund or an exchange I need a receipt and a confirmation from the store.

zamjena (feminine, ends in -a) changes in the accusative: zamjena → zamjenu
povrat (masculine, consonant ending) often looks the same in nominative and accusative when it’s inanimate: povrat → povrat (same form)

Is povrat the usual word for “return (an item)”? Could I also say something else?

Povrat is common, especially in shop/administrative language (e.g., povrat robe = return of goods). Other possibilities you may see:

vraćanje (robe) = returning (goods) (more “process”-sounding, everyday)
povrat novca = refund (return of money)

Why does it say trebam and not mi treba?

Both are possible, but they’re structured differently:

Trebam račun i potvrdu. = I need a receipt and a confirmation. (subject I is implied in the verb)
Treba mi račun i potvrda. = A receipt and a confirmation are needed for me. (more impersonal/common in some contexts)

In your sentence, trebam is perfectly natural and direct.

Why is it račun i potvrdu—why isn’t it račun i potvrda?

Because they are direct objects of trebam, so they should be in the accusative.

račun (masc. inanimate) has accusative = nominative, so it stays račun
potvrda (fem.) changes: potvrdu is accusative

So: trebam (što?) račun i potvrdu.

What exactly does račun mean in Croatian—receipt or bill?

In a store context, račun normally means receipt (proof of purchase). It can also mean bill/invoice depending on context (restaurants, services, business), but for returns/exchanges it’s typically the receipt.

What does potvrda mean here? Is it something different from račun?

Potvrda means confirmation/certificate/proof. In this context it could mean an additional document or proof (depending on the store): for example a stamped confirmation, a warranty-related slip, an order confirmation, etc.
If you specifically mean “proof of purchase,” Croatians often just say račun, or more explicitly:

račun i potvrda o kupnji = receipt and proof/confirmation of purchase

Why is it iz trgovine and not od trgovine?

Both can be used, but they emphasize slightly different ideas:

iz trgovine = from the store in the sense of origin/source (the document comes out of that store)
od trgovine = from the store in the sense of from that party/company

For a document/receipt, iz trgovine is very common.

Why is it trgovine and not trgovina?

Because after iz (meaning from/out of), Croatian uses the genitive case.

trgovina (nominative)
iz trgovine (genitive) = from the store

Is the word order flexible? Could I say Trebam račun i potvrdu za povrat ili zamjenu?

Yes. Croatian word order is flexible, and both are natural:

Za povrat ili zamjenu trebam račun i potvrdu iz trgovine. (topic/purpose first—sounds slightly more “formal/organized”)
Trebam račun i potvrdu iz trgovine za povrat ili zamjenu. (more neutral, often closer to English order)

Does ili mean an “either/or” choice here?

Yes. povrat ili zamjenu means a return or an exchange—one of those options. If you wanted to strongly mark “either…or…”, you could also use ili…ili… in some contexts, but here simple ili is standard.

How do I pronounce the tricky parts like zamjenu and potvrdu?

Approximate pronunciation (stress can vary by region, but this will be understood):

zamjenu: zahm-YEH-noo (the mj is a combined sound, like my said quickly)
potvrdu: pot-VR-doo (the vr cluster is pronounced together)
račun: RAH-choon (with č like ch in church)