This is a more specific question than Why is it sometimes difficult to get a receipt for a flight as it focuses on one specific site, but the premise is the same, and for consistency in this question, assume a JetBlue flight booked through Priceline. (JetBlue says you have to get the receipt from Priceline). How can one get a receipt?
Logging in to Priceline, one can go to My Trips, click on a trip to see details, and there's even a top menu item "Print Itinerary and Receipt." However, this link just prints an itinerary listing the travel details and trip cost, with no indication or acknowledgement or showing of payment.