I have sent hundreds and hundred of 'letters' overseas without a customs form. Inside were bulky items within the 20mm requirement, all in normal sized envelopes.
I started off doing it the 'proper' way, then I got sick of having to drive to the post office, line up for ages and show ID, all just to post a letter OS. So I started skipping the customs form and just dropped it into my Red Street post box at the end of my street.
In those few years every customer received their item. I was posting them to the US and Canada. Items worth around $50.
So just something to keep in mind if you are sick of the hassle.