Upload & Sell: Off
Because oversize mat boards (and museum glass) are special orders, you may have to order several sheets in one order.
I think the reason is that it requires special packaging and handling for even one mat to keep it flat and not bent or crunched. Thus shipping costs soon add up for it to be prohibitive for a single mat.
However, a local framer might have one on-hand or be willing to let you piggyback one of his orders.