I would suggest printing out a copy of your ride confirmation, with all the details. I would not be happy with a much shortened grocery stop. If you book a particular company and they substitute another company, fine. But, the subbing company needs to honor the original contract.
I used one company that told me, repeatedly, that I would be able to make a short grocery stop on Thanksgiving morning, around noonish. I kept asking, to be sure, if it was going to be an issue. If it was, I would bring the stuff I wanted with me. No, I was told three times that I would be able to make a stop. Well, the driver picked us up (after having to go and look for him), and then told me there were no stores open on Thanksgiving!!! Nope, no stop. I was not happy.
So....be sure to have all your details in writing with you. Just to be on the safe side.