Did you see a popup message with the offer when you were logging in to the game?
If so, you have to click on the popup message to navigate in to the offer on the payment page.
If you navigate in to the payment page another way, it might not have the offer.
If you want to message Billing Support and ask, start on the
Didn't receive my Treasure Hunter Keys
support page and use the 'Contact Us' button to navigate to a message form.