Tricky situation...

So, my daughter is 8 months old. Recently the doctor measured her and she was 31 inches long. I freaked out because that puts her over the height limit for her car seat. I measured her at home and got closer to 29.5, but it's still close.

We need to upgrade to a convertible car seat for her. BUT, I don't know what to do about going to the store and such. I'm a SAHM and I also have a toddler. We run a lot of errands. When my toddler was old enough for a convertible car seat, she sat really well, so she was able to sit in the cart easily.

My infant doesn't sit great. She still falls forward sometimes. Typically, I would just wear her, but she JUST had abdominal surgery and I'm afraid that wouldn't be comfortable.

Thoughts? Troubleshooting?

Edit: Sorry, I wasn't very clear. Haha. I'm concerned about the transition. I have no issue going to buy a car seat, I'm worried about once I DO and how errands will look since she doesn't sit well and I can't wear her.