Call you TA Monday morning and tell her that unless the bus happens to NOT have any stairs, that she had better change your arrangements to Super Shuttle.
Super Shuttle is basically a regular van. The Airport Bus is a motorcoach bus with probably 4-6 stairs to climb up. So basically it depends on if your mobility will comfortably allow you to get up those stairs.
I do motorcoach tours, and some of my people have mobility issues. I have some that can no longer travel by bus because of the stairs.
If you don't think you'd be able to manage the stairs, then please call you
travel agent and explain that an error has been made and you have mistakenly been put on the BUS instead of the SHUTTLE, as was arranged. If she says that there's nothing she can do, smile and say that you're really very sorry but your disability just will not allow for you to navigate the stairs on the bus and that perhaps you really should be speaking to the manager.
I don't know what vendor your TA has booked your package through, but I know that most of them will courier documents out for next day delivery. Unless the vendor feels particularly generous they will probably charge for changing the transfers from the bus to Super Shuttle, but you need to insist that the agency should eat that cost if there is one.
You're using a scooter. You have an obvious mobility problem. You were told that you were being put on the *shuttle* and had no reason to expect that you were being booked onto a bus that is not handicapped accessible.
And while in some cases "shuttle" and "transfers" are somewhat interchangeable, and your TA might argue that by "shuttle" she simply meant that she was arranging transfers... when you're dealing with 2 companies one of which has "shuttle" in the name and the other doesn't as a TA she should be aware that if she says "shuttle" and one company is named "shuttle" then her client is very likely to believe that's the company they're being booked with.