Car seat dilemma

Yep, his height is maxed out for the infant seat. They make someinfant/toddler ones that will convert from rear facing to front facing that go up to 40 pounds. I just didn't want to have to buy one yet :( My big issue is that he can't sit up on his own yet (almost, but not quite); so if I get the bigger seat, I can't just pull it out and carry him into the store. That means I don't know what I will do with him since he can't sit on his own (think about grocery shopping, eating out, etc, basically everything I do involves carrying that seat with us). Any thoughts???

For the grocery shopping, I have considered using the carts in the store with the built in seats (but just covering them with something to protect from the germs). That is especially true since my favorite grocery store just ordered new carts that DON'T hold carseats any longer. I talked to the manager (and she had an infant so she totally understood), but she didn't know if they were going to keep any of the old carts. Ugh! I know that was a tangent but I mean, sheesh, the carts should fit the damn carseats. You think that they would have considered that before they ordered them!