3824952 - How did a Frontier flight get issued for a different fare once it is instant purchase?

When the traveler booked the reservation he was shown a price of XXX for the one way booking he made, and he was charged YYY for his reservation by the time we sent over the invoice. 

The airline booked was Frontier. Frontier is an instant purchase carrier, and by the time we get the PNR in the GDS, the reservation was already paid for. In our file finishing that came over from Concur it showed YYY. We checked Frontier directly as well and it shows YYY. 

Can you explain why the traveler was shown XXX when he was making his booking?

User selected a Frontier flight for one fare and noticed it was issued for a different / higher fare. , KBA , CON-CA-ADM , Administration , Problem