I'm an international customer and here's my tuppence worth!For me, money for "fun" things like fabric is a treat and I have to get the best value I can for it. I check the postage charges of the sellers and how many yards of fabric they can get into the enveolpe. I've always had 8 or 9 yards in a package and have never had a problem with the packages.
If only part of an order that I'd made was available, I would definitely want to know in advance to give me the option of either changing my order (I'd fill my envelope up with other fabric) or cancel it altogether. To me, I'm always working out the cost per yard including shipping, so to have an envelope half empty puts up the per yard cost of the fabric.