Your receipt number is at the top of your receipt and in the confirmation e-mail we sent when you bought Priority Review. It starts with INV- (queue boosts start with QBST-, and charges by payment link or terminal with TRM-).
You can also sign in and open My purchases (/applicant/purchases): every order is listed with its number and receipt.
Can't find it? Send the change request anyway with the e-mail you used — a person will look it up.
Something not right with a charge?
Send a change request: billing mistakes our records prove are refunded automatically, and a person answers the rest within two business days.