I agree. My parents pay for a spot, as opposed to hourly wage, as I see it. I had the same situation during the school year. Mom only needed care for an hour or so daily. Be prepared for them to stretch out the length of time that the child is in your care. That's what happened with us; gradually a 4 o'clock pick up became 4:30, almost 5. Not a big deal, but a little irritating.
What about for full days? In my dayhome, parents paid a flat fee for the B/A care, and an additional daily fee for any holiday or prof development days ($10, which basically just covered the extra food). Perhaps if this mom will need those full days, you can include them in the cost, IF she agrees that the 40 min daily will be the standard timeframe. If she needs to pick up later, consistently, then I would go back to charging daily.
Keep in mind, too, that 4 pm in the spring, summer and fall can turn into an extra half hour to an hour later in the winter. At least where I live it does!! lol