Bummer you had to pay a customs charge. I too have been stung with a charge in the past.
Unfortunately the charge has nothing to do with Spoonflower (or myself) and is impossed by the UK postal service. I am guessing that it is an attempt to keep people buying items locally? who knows? The only way around paying a customs chrage is by making purchases under 40 pounds or declaring the value as lower than 40 pounds, which would be fraud if not true.
A matter to bring up with your local member for parliament perhaps?