The price at both sites seems right for 90" batting. It comes on the roll folded in half so the roll is really 45" long but once you cut off what you need you open it up to the full 90".
The shipping on the Amazon site definitely seems to low. So I would go with the JoAnn's sale price and take advantage of the free shipping if you can. Roll batting is considered "oversize" by UPS and they charge based on dimensional weight rather than actual weight. So JoAnn's may restrict the use of the free shipping code on this particular item. However Wanda did post she took advantage of it recently so I say go for it before they realize how much they are losing on the shipping charges! I just ordered a bunch of batting direct from Hobbs. I have a resale certificate so can order direct but I had to order a bunch of batting (minimum of 4 rolls or carton combination). A couple of local LAers got together with me to put in the order. Anyway the shipping charges just about gave me a siezure. It was still cheaper than if I had ordered from CT when they had their 30% off sale and free shipping. But that is because I am buying wholesale. The shipping was a rude awakening! I even had enough to ship common carrier which was a far cry cheaper than UPS.