Question about extended stays and pricing/payment?

I recently booked someone's dog for a 5 day stay. It was $335. However plans changed for this person and they wanted to book their dog for two more days. I utilized the extended stay option request which came out to 7 days for $469. However the request I sent out prompted the person to pay the entire $469 amount on top of what she already paid which was $335. I thought Rover would just have her pay the difference. Why would the site make her pay the entire amount instead of just the difference? That doesn't make sense.