but why can t you just put the couplers in an envelope, the necessary stamps then in the mail box, as a letter, and voila ?
For a standard envelope (like you would send a letter or a bank check in) that won't work. They have "flattening machines" (I don't know the actual name) that those run though and it will literally squeeze small parts right out the end of the envelope.
They can go into small padded envelopes, but I cannot answer why that would be so prohibitively expensive. Many sellers use shipping as a cost center so sometimes it is hard to determine what is an actual shipping issue versus something else.
I will say that shipping internationally is just a challenge. Even US to Canada, the prices vary wildly week to week. It's not uncommon to get a shipping estimate and then have it double when you try to actually ship it.