The thoughts I had while reading your post question. I do not want to read the responses until after I have made mine. In my response please do not think I am being gruff.
Customs has a dim view of one recipient and/or address receiving multiple shipments from the same sender and/or address shipped on the same date. If you arouse customs suspisions each shipment the sender sends into that country may be held for inspection until the suspisions go away. In the same turn, all shipments for that recipient may be held for inspection until any suspisions go away.
Some quilters use wholecloth. Seems like cutting the fabric in half could result in a problem in the layout of the quilt.
If you only ship in flat rate envelopes your shopping cart should note it's limitation of xx weight and/or xx yards.
If you offer the option of shipping total yardage in a larger box the shopping card should so note with emphasis on the 'additional shipping charges'.