It's an eBay thing that started sometime last year. I have a seller friend who has been subjected to this.
It is meant - so eBay says - to provide a means of tracking orders by having the postal service scan the address (using OCR?) and recording the code which, when passed on to eBay, allows eBay to keep an eye on orders even when a paid tracking option from the postal service was not used.
I knew we had it in Australia, but I didn't know it was used elsewhere.
The code is 7 seemingly random alphanumeric characters - where the alphabetic are always in lowercase. There is likely some algorithm that drives their generation as I have seen multiple separate purchases from one customer within a short period having 5 or 6 characters in common.
I have no idea as to whether what eBay has said is actually what happens - or whether anything happens. Even if nothing is done with it yet, I have little doubt it might in the future.