To reset the oil maintenance light press the display until it reads the odometer reading, turn the power switch off, press the power switch to ignition on while holding down the trip meter reset button, keep the trip reset button held down for about 10 seconds , the display will read 000000 and then the light will go off.
I would highly recommend taking your Prius into another shop. You technician should have absolutely no problem performing the service and having everything normal and ready for you once he is done.
It is a signal that you are not receiving the service you should if the job is not 100% complete when the car is returned to you.
Obviously... There's absolutely no reason to pay $89+ to have your oil changed - It's simply a dealer rip-off. Major mileage service calls goto the dealer. Bought my tires at Costco - Free rotation for life. I change my own cabin (dealer wants $35, I pay $4.50), air filters (dealer wants $75, I pay $11). Enough said!