Car Seat HELP!!

Ok so my son is nearly 8 weeks old. We purchased an infant seat/travel system before he was born and I was pretty happy with it but then I started thinking about convertible car seats. I discussed with quite a few moms I know the pros and cons and decided to wait on splurging on a convertible seat. But I think it’s time.

We are about to make an 8 hour drive to look at property we are buying, and many times a year we drive 13+ hours to Texas. He seems so uncomfortable in his little Chicco infant seat- it hunches down his shoulders despite me adjusting it appropriately, there’s really no padding and he squirms and squirms. I don’t want him to ride that long being uncomfortable. I have a few seats in mind but my question is for those who use this type of seat especially in babes this young- what do you do if you go in somewhere? Like if I go into a store quick to grab something? I love the ease of the infant seat for that reason, if he’s sleeping I don’t wake him by taking him out of the car. What do other mommies do with permanent seats?!?

Thanks!!