To get a refund, please return the product unopened.

Literal

Refund [object-を] receive [purpose-には], product [topic-は] unopened [possessive-の] [condition-まま] return please.

払い戻しを受ける means 'to receive a refund.' 未開封のまま means 'in an unopened state' — 未 (not yet) + 開封 (opening) + のまま (as-is). A formal customer service instruction.