You could ticket it, print the receipt, cancel immediately, and then book again with the travel credit. Then use the first receipt.

Of if using the 'real' receipt, you want to be reimbursed on the "Total Per Passenger". The "Total" is what is charged to the credit card, but the 'Total Per Passenger' is the actual ticket cost when you add the elements above it. I'm cropping a bit, but this is what it looks like.