Za povrat ili zamjenu trebam račun i potvrdu iz trgovine.

Here za means for / for the purpose of. With this meaning, za normally takes the accusative case.

• za povrat = for a return
• za zamjenu = for an exchange (note zamjenu is accusative of zamjena)

Because they’re different noun types:

• 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)

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)

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.

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.

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.

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

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.

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

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

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)

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.

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)