WOW! First I am glad to see that moms on here are actually being HONEST about this type of scenario! We all know that we should NEVER leave are babies/kids alone in the car. However, we live in the real world and sometimes we have to weigh our options and deal with the consequences, if there are any.
In the future you could ask your spouse to ask the in-laws to help you in these situations.. Watch the car while you run upstairs etc. Seriously, of course you would put the baby in last but how many times do we remember EVERYTHING we need to bring?
When we were in the city (Albany Park) I had my son in his seat carried him out, had to go back to the condo on the way back I slipped on an ice patch and totally messed up my back...and he fell too. I ensured he did not fall so the car seat landed gently on me! So I can see why you would be scared if you are still recovering and shouldn't be lifting!!
Once we had 2 children 13 months apart I tried my best to carry them both, this totally messed up my back too!! I could not always do it One in the carrier and one on my hip. IT was VERY difficult when we had child care and lived int he city because in they city you don't always get a front row spot and I would take one to the sidewalk, leave him get back to the car to get no. 2 and try to walk them to the destination... it SUCKED!
I had a scenario in a bathroom at a rest area where they ran under the stall and I had to chase them in different directions. One child hid in another stall while the other ran out to the main area! There I was trying to pull up my pants...and get my kids I was terrified!!
So it isn't easy to do any errands with kids...do the best you can.
I always laugh hearing from our p's "well we were able to take care of our kids on our own" blah blah... Of course they DID leave their kids in the car and weren't nearly as criticized and ridiculed as we are now!!
I wanted to add that you if you are still recovering you do need to take care of yourself!! If that means no lifting don't lift... Enlist help wherever you can...Your baby needs YOU to be in good health!!
I hope this helps you... Good Luck