Don’t tell anyone this, but while buses in my neighbourhood don’t take even folding bikes, as policy, I’ve carried my Strida 3 either before the policy applied or because they didn’t figure out it’s a bike (folding in the handlebars and pedals makes the difference), or, more recently, just by making a large plastic sack… out of a couple of smaller plastic sacks… the most effective, fastest, design being apparently “tall”, i.e. to fit over the top of the upright folded Strida and down to the ground. Tape two bags together, mouth to mouth if you see what I mean, and cut the resulting thing open at one end - and then put tape around the new bag rim to strengthen it.
I thought a larger bag that I could slip Strida into sideways would be quicker, but it wasn’t.
Use the thickest large bags you can find on sale, such as “rubble sacks” (problem: they don’t quite come in Strida width, usually), and you also can roll up the bag and tuck it into the bike’s little luggage rack. However, the bag may not last for very many uses: I’ve put a lot of tape repairs onto mine after only a handful of uses. Still, they’re cheap, i.e. replaceable.
Oh, and best try not to be seen by your driver folding or unfolding the bike, too.
The “genuine” bag may be worth the money. I’ll guess that the non-airline model is more hardwearing?