I'm after some advice as I'm sure somebody else here has done this before....

I recently bought a new rucksack from Victorinox - it was delivered by DHL and apparently came direct from Germany. I'm not very happy with the bag and would like to return it - I've been through the Victorinox return process, which in turn directs me to the DHL returns page to complete the details. It says I have to complete a Customs Declaration (CN23) and attach it to the outside in a clear document wallet etc.

First time I'm sending something to the EU, my question is would I better off sticking an A4 document wallet to the package (it's certainly large enough) - so the whole CN23 is visible or should I fold it and pop it in a smaller wallet?