Great service but wrong billing!

Like: Efficient staff, car in great condition and the holiday was made very pleasant with a car driving in Melbourne and surrounding areas.
Dislike: Wrong billing? I made a reservation for a Toyota Rav 4 for the period between 26 to 29 Aug 18. The reference number was 63103417 and confirmation number was 38357487SGD. The quote was AUD 303.72. I made a payment of AUD 60 as a deposit, and the amount due at pick up at the airport was AUD 243.72. At the airport, I was asked if I wanted an upgrade to a Toyota Kleuger which came with a GPS at approximately AUD 60. I agreed, and the final bill came up to be AUD 359.39 and I was charged this amount on my credit card. Shouldn't I be paying AUD 299.39, having considered that I already paid AUD 60 as a deposit? The bill number issued by Budget upon return of car was 317692550 and the reservation number was 38357487-SG-OH.