I am currently an inhome licensed provider...
Centers can not be as flexible with the rules as a home daycare can be. Other wise EVERYONE would take advantage of them and they would go out of business quickly.
I can see fault with the center in this because THEY knew what time he usually gets picked up and after about 5 minutes past that time should have been on the phone WITH the bus commpany, school, or you.
It is not YOUR fault that the driver got sick and they had to use a sub... (my mom drives school busses and so I know all about the issues with subbing for someone who probibly doesn't have their route thoughrouly charted)
I would contact 4C www.mi4c.org/ and ask them for a list of local providers in your area. Then when you are questioning them ask them about their charging scale. What they will charge for full day and hourly for your son... Perhaps you will be able to find one close enough to your sons school that bussing would not be an issue.
I am assuming that you will still be working this summer and would want to find some full time care anyways for both of them...
If you do decide to switch then let the center know that the reason you are switching is because of the " over charging" for things beyond your control.