I am in the UK and frequently buy fabric from US online. I try to keep the value below about $35 so that I do not have to pay duty, but it means I do not always end up filling the envelope. Usually I buy fabrics I have either already seen in a store, so know what they will look like; or I buy from specific collections. I have never been disappointed. I have also bought fabric in UK from a local shop and then bought what I want for the backing online, so it works out a lot cheaper when I need several yards of one particular fabric.
Best online seller I have come across is www.debsews2.com. She also sells via EBay. She is incredibly helpful.
Pamela