I think one of the major hurdles to overcome with online mattress sales is perception of shipping. I do a lot of online shopping and have no issues with it. If I were in the market for a mattress, my first thought about online mattress sales would be "It's going to be an arm and a leg just to ship it." and would perhaps not even start the online process. Just my opinion. (From Canada so Amazon Prime and free 2 day shipping doesn't apply to me).
It really is for sure. We've had to get a special machine to package the mattress in a smaller box but we're still paying for what's called Dimensional weight. It's large enough that we aren't being billed for the weight but rather the size.
Luckily we've managed so far though and have been able to streamline some of the process. I imagine it will become easier as we grow.