Shipping cost is about 14.50 for a flat rate envelope to Europe. And filling out a customs form doesn't take much more time than a regular shipping label for us, because we do so many, but it may be that Hancocks has an automated system for regular shipping labels but not for customs forms, so they feel that that requires an extra premium. This is probably the reason a lot of larger outlets won't even do international shipping.
Having said all that, putting 6 yards into a flat rate envelope requires a lot of extra time in careful folding and packing to make it barely fit and there is also a lot of extra messing around with it. And the fabric is not well-protected when it's all done because there is no room for good protective packaging.
Our maximum for flat rate envelope shipping is 4 yds, domestic or int'l because we use heavy duty inner packaging to avoid damage, and we don't want to have our packages broken, returned because they're overstuffed or spend an extra half-hour reconstructing those cheap cardboard envelopes with tape.
With a 6-10 yard order, it'll still fit into 1st class parcel (4 pounds max), (which lots of shops won't use because you have to supply your own packaging), get there just as fast and be just as cheap or within a few dollars of flat rate envelopes.
Even with higher shipping costs though, you really need to compare the total cost of purchase online with what you would have to pay for the same thing locally, including taxes, shipping and even transportation to get to your local stores. This applies to domestic or international online shopping.