I guess my first question (and I haven't stopped to read all the posts) would be why are you making available for purchase fabrics you don't have on hand?
My background is supply management for hospitals, so I am familiar with how things work between customer and distributor.
When we realize that cotton is at a premium right now, I'd be anticipating you will have backorders more frequently.
I just wouldn't offer what I don't have. What you might do is make it clear on your website those fabrics that are "coming in Month XX, advance orders welcome" when you post them up on your site and then if the customer orders it, another pop-up explains that additional shipping would apply and offer the opportunity to have the order held until it arrives.
I use one or two online suppliers who let me put instructions about backorders and shipping, and it works well. I've asked them to change their shipping method if my quantity makes their standard method difficult, with the comment that "lower shipping cost is appreciated".
Hope that's helpful.