I have purchased a few quilt kits. I purchased 2 identical kits to give as Christmas gifts. Both kits were short on one particular fabric. I contacted the reputable site that I had purchased them from and they didn't have any of the fabric in stock but were willing to purchase it for me online and send it to me at no charge. I contacted Robert Kaufman as they had packaged the quilt kits themselves. They sent me more than enough fabric to finish both quilts at no charge.
I don't buy the backing at the same time I buy the kit. The backing of a quilt is an opportunity for me to add just another element to the quilt and I don't decide how I will do it until the top is done. I have only purchased one kit for a large wall hanging that actually came with the backing fabric.
I have to be really in love with the fabrics in the kit to buy it. I enjoy the process of picking my fabrics and I don't want to make a quilt that many other people will have the exact same one, even though I may never see it!