Welcome to the sometimes unstable business of cutting lawns. I guess that's why it's probably best to always have more customers than you really need, because people will flake out on you, cancel appointments, use a different person, etc.
But I definitely wouldn't deal with those people who wait until the grass is way too high before trying to get it cut. I charge people like that extra. Especially since it's going to take you extra time, extra gas, and extra work to cut it.