Generic 50 x C5 A5 PIP Box Shipping Mail Postal Large Letter Boxes - Size: 235x165x22mm - Sold by Youthstar

Yet if any seller offers royal mail I'll use it even if it's more expensive. This is the only company which has never ever lost absolutely anything of mine and this includes not tracked letters like bank/credit card statements.
About others:
DPD lost my bicycle last month!
Evri courier broke into my shed and left my parcel there (no info left)
Yodel threw my parcel into my neighbours garden across the street...

Previously weight was the deciding factor.
Then size & weight became a factor
Eg. Companies used to send 'letters' with pens inside them. Low weight, so cheap to post. However their buik meant they had to be hand sorted by RM.
Then they became large letters.
This was introduced in 2006. Nearly 20 years later and people still act like its a new concept

You know, it was only until reading your comment that I realised it wasn’t! hahahahaha

Use these all the time, you get used to foding them after a while. No issues at all with RM accepting them.

this stops you sending things to friends and family and them getting a card though rhe post saying "come pay £1.60 to get your packet - the sender didnt use enough stamps"

My mother in law always used to do this. She'd see a newspaper clipping, a packet of free seeds or maybe she had done a bit of knitting she didnt want to keep and she always said "Royal mail will send anything for the cost of a first class stamp"... they never did you had to take 2 hours of your own time and a couple of quid to get a piece of junk from the post office.

Long story short - when you eventually need put in a home this sizer will have been a small price to pay to have avoided slowly winding your son in law up over the previous 40 years.

So annoying find some cartons sold as 25mm compliant are actually over 25mm. No fix & card suppliers seem allowed a small percent description error owing to nature of material. Trial & error find a reliable supplier but it can vary by batch.