It is not possible by definition to be CAT C for straight in and CAT D for circling, unless your airline has wrongly interpreted Aircraft Category definition.
What determines the aircraft category in which you fall in, is the Vat , which is the speed at threshold called also Vref
You could be doing 160 kts on final or 180 kts in a circling , you are still in C category as long as your Vref is within 121 -140 kts at threshold.
In ICAO document 8168 , There is a range of speeds for each aircraft category depending on the position you are in the approach:
As an exemple for CAT C :
Vat: 121-140 kts
Range of final approach speed:115 - 160 kts
Circling : max speed 180 kts
Initial approach: 160-240 kts
Etc..
So if you are doing 180 kt during circling approach you are still not in D category as , your Vref will still be in the C category when you cross the threshold
Unless you decide to land at 180 , in that case you will be in E !
Also landing fees, are based on Aircraft Category. The higher the category , the higher the landing fees.
But you will not pay a higher price if you were doing 180 kt in circling , as your threshold speed was still the same.