I just had a shower this past weekend and it was out of town (about a 5 hour drive) and my guests were pretty aware of travel with bigger things. The only big item that was given at the shower was the pack n play which fit easily in my trunk. I did however had a couple of aunts print a picture of what they wanted to buy (the carseats) and then inside the card with the picture was a gift card to get them.
I also attended a shower for my cousin earlier this year in Texas and she lives in Nevada and people also seemed to understand she was flying so big things were harder. I know she got a couple of big things but she just paid to ship them to her house and after talking with her it didn't cost her too much to ship them.
The other idea is great as well, return things and get the store credit then re-buy when you get home.