Interesting
I booked my upcoming trip via Expedia and did not see any extra charges.Could the $4.50 be a booking fee? I don't think this would be plausible as there should be no additional booking fees. I would call your credit card company to see exactly what the charge is for.
If you call Expedia customer service inquire to see what the $4.50 is all about. I would print out your itinerary with payment details as backup to show that you did not authorize the extra $4.50. You should only pay the amount as shown on the receipt.
Good luck.