When I worked retail the schedule was posted on Thursday for the following week and that included not knowing till Thursday if you worked that weekend or not so it is entirely possible that they don't know.

What you could do is take the average of what they have been working and make that the contract in terms of fees that are required and then there would be adjustments for additional time but nothing off is the minimum time isn't used. At least you would have an income you could count on.