You can fill in a complaint because I see they already made a return shipping label.
Another possibility is you putting a small number on it? A pro forma rate, let's say the price you paid for the packaging?

I have already called the postal services a couple of times. They stay friendly but I know they are doing nothing.
Just one of the ten thousand shipments that are laying around and they want to make money on.
They are just holding it because returning the goods will cost them money instead of making money.

Don't worry about anything, I would go for the simple solution and let them deliver here in Belgium.

Jo