How can I get a copy of my purchase receipt?

I have a problem with my purchase! On September 5, I bought a computer in the App Store, but when I received the order, I saw that the RAM is not enough for me! I want to change my computer to another one with more memory! But there was a problem because I deleted all messages from the App Store. Is the order number sufficient for an exchange or can you send me a copy of the receipt?

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