I used a black garden waste bag (slightly bigger than a bin liner) for our car seat. I put an extra one or 2 in there to use on the way back. Car seat arrived in pristine condition.
Our pushchair is a light colour (it was cheaper than more practical darker colour) so I bought a bag off Amazon for it. It's a Sunshine Kids car seat bag but it actually fits our pushchair and even had room to put in a packet of nappies and our coats, freeing up precious room in my suitcase for those clothes that I need to take but probably won't wear
. The bag won't fit all types of pushchair so check dimensions carefully before purchasing, mine's a Baby Jogger city mini.