I have ordered a gazillion times from the US. All kinds of items, fabric, thread, patterns, quilt tops, etc. I have never paid any duty/custom fees. And I've purchased from individual people, either privately or off ebay/etsy.
I've ordered form Connecting Threads, The Quilting Hen, Fat Quarter Shop, & various other on-line shops & have been totally happy. I often order when there is a shipping deal for international. It doesn't happen often, but when it does, look out!!!!!
I understand that the shops need to add that disclaimer, just in case, because it can happen.
I hope this helps.
Maureen