Hello, Royal Mail Customer Services employee here
I'm sorry to hear of the problems a lot of you've had.
Hi mxfun, as a number of people have said, it's sounding like the thickness has caused the problem. It's worth mentioning that mail has to fall through the size guide under its own weight to ensure that it isnt surcharged. (And no its not the best when Post Offices have their size guides bolted to the desk in the vertical position)
I don't know how the item was packed, but if each filter was 3mm and they moved in transit, the item potentially could be at least 6mm thick, which would automatically make it a Large Letter.
With regards to what will happen to the item, from our own blurb;
if the item displayed the full address details of the recipient, it is likely that our delivery colleagues will still process the item but will request payment of a 'surcharge fee' from the recipient: the amount charged will include a £1 charge to cover our extra handling costs, plus the amount of the missing required postage (this will also depend on the weight of the item). The receiving Delivery Office will keep the item and deliver a card to the addressee to inform them that an underpaid item of mail is being held. This card will also provide instructions on how they can pay the surcharge and receive the item of mail.
The recipients Delivery Office will hold the item for 18 days, so it might be worth asking the recipient if they have received a grey and white fee to pay card.
I noticed that you mentioned that you didnt put your return address on the outside of the item.
If there was one thing I would urge people to do; it would be to put at least your house number and full postcode on the outside of everything you send (full address would be better), no matter how mundane.
If an item doesnt have a visible return address, they are sent to our National Returns Centre (NRC) in Belfast think Warehouse 13 from the show of the same name!
The NRC is the only place in the country were legally allowed to open mail. There they open it under secure conditions constant CCTV coverage - purely to try and find a return address inside the packaging.
If none is found and its not deemed to be monetarily or sentimentally valuable, it will be destroyed; purely down to the amount of stuff they handle.
As such, if you put a return address on the outside, it avoids that whole process.
The last thing I would say is that mail doesnt just fall of the face of the earth. Unfortunately thats not to say that there arent a few bad eggs; but delays and losses are often down to poor, incorrect or incomplete addressing, wrong postage or genuine Royal Mail error.
Royal Mail hat off.
Hope thats of some help!