From research I've done, I've seen most people charge an hourly rate (or a per/task rate) for errands, and for something such as gift or grocery shopping, they may charge 10% of the total bill.

For instance, they buy all the groceries on the cutomer's list, then take 10% of the grocery bill, and the customer pays that in addition to the hourly rate for the shopping.

Is this how it usually works?

Thanks for any advice ~