I take the order page plus 2 blank sheets of paper and then I put the largest denominations of currency I can get on top and fold them up into a legal sized envelope.
I usually print the address to make sure a blind man can read it and get it right. I seal the envelope normally, no extra tape or anything, and I wait with everyone else in the post office line.
When you get to the window just tell them that you need to mail this letter to England,pay them and leave.
Once a clerk asked me what was happening in England and I said it was for a birthday gift for my wife that I saw on E-Bay. Turns out that he does a lot of E-Bay buying and selling himself and he went on to the next person while my letter went on it's way.
Out of 15 or so orders, I have only had one not go the distance, but that was somebody other than SeedsDirect, they have been 100% so far.