Data overage. Can I avoid the fees by increasing data cap m month?

Hi, I'm in the middle of a billing cycle and have a 5GB wireless data plan but I've gone over by 3GB already. $30 total in overages. If I increase my data cap to 15GBP now will i still incur the $10x3x per GB overage fee. Or will my new increase to 15GB cover it?

Thanks for your help.

George