Take a picture of both carseats side by side. Wrap it in a small box and put it in the gift bag with all the other stuff. Personally, I'd put it on the bottom. Or you could even print it out on computer paper and put it in a empty paper towel roll and wrap that and add to the gift bag. No way would I cart those huge boxes to the shower. You could always add a note that you will drop them off at her house when it is convenient for her.
This, I like the picture idea. It would be hard bringing home the big boxes.