When I ordered my trailer, I supplied the fabric, counter top edgings and vinyl flooring I specifically wanted. When picking up my trailer, I was given all the leftover materials. The vinyl flooring was quite expensive so it was nice to get it back. In fact, I probably have enough to do the bathroom in my brothers house! However, being to the factory numerous times, it's pretty obvious there's not a lot of room available to store extra materials let alone keep track of it. I guess as long as you request AT THE TIME OF YOUR BUILD that you'd like the leftover materials back, I'm sure they'd comply.
Otherwise they could be left holding onto stuff indefinitely. We don't want to ruin a good thing. How many other trailer companies out there allow you to have such choices?