I'm answering strickly from my point of view as a customer. If I ordered A and B and was advised that you could ship A now, but B is not coming in for an unknown time I would request that you send A now. You then advise me that there would be an additional shipping charge when B arrives. Personally if I wanted B bad enough to order it I would agree to the additional shipping charge. I would want to be kept informed throughout the entire order. If you knowingly offered B when it was not going to be available then I would be upset. If, however you advise that B is in a backordered status and I still want the fabric then it is my decision to wait and pay the extra shipping charge or ask you to delete B from my order.
As I do a great deal of business on the internet I do run into the backorder situation, or demand was greater than the supply, and must make the determination if I want the item sufficiently enough to warrant paying additional shipping. Nine times out of ten if I am kept advised by the vendor then I am willing to wait and pay the additional shipping as I understand it is a part of doing business. The only time I will get irate is if the vendor knowlingly "snows" me. That will end the relationship. Communication is key to good customer service.