Line manager fee?

So unfortunately, I had a back to school parks dept. event scheduled for today that was canceled due to rain

However, on the bright side, the woman who hired me asked me to do two other events! Yay

I didn't include a line manager fee in the contract for today, but had mentioned to her back then that in the future I would. So, my current rate is $80/hr. Does $20/hr for a line manager sound like too much? Should I make it $15?

Thanks for any and all advice!
Julie

My kids know what I make, and have a hard time unless I pay them decent...guess it depends on who works for you. I would ask for 20 and if they dicker, go to 15. Let them know it is a TRAINED assistant,
who is familiar with the routine, designs, etc.