You would need to fashion some sort of brace for it, in case it takes a light beating so it simply doesn't snap off. What GPU is it? If its really small and bolted down it won't be an issue, some of the larger ones I wouldn't even attempt.
So yea, you could always use cardboard, like make a small "mold" (cutting and folding) to be placed inside the case, with bubble wrap? I dunno, the thought of it kinda makes me worried.
I'm sure if you simply wrapped it in bubble wrap and even tapped it to the inside of the case or zip tied it down or something and explained how dangerous it could be to ship it installed (if its a large card) and I'm sure they would understand, or perhaps thank you, for being so thoughful. No1 wants to receive a shipped computer with the GPU snapped off from the slot...
Would be easier to make a suggestion knowing the size of the card though...
Edit> Or better, and simpler, do what JRux suggested.