Invoice Recepit

Goodnight, I was sent a gift card and a little while after I was sent an invoice thanking me for shopping at apple, saying that if I've already paid for my purchase to keep the invoice receipt for record and that if I needed to send apple a payment, reference the invoice number. The email said where the gift card was shipped to and from, invoice number, date, the web order number, terms (credit card) etc. but I am so confused. I tried calling apple but it was no help. Can anyone offer assistance please?