Crikey, I'm late to this...
If you didn't get sorted out, here's a bit of info..
There are two types the thickest are about 5mm at the thickest part of the sleeve (the boarder) and the thinner ones are maybe 3mm.
Selwyns were taken over a couple of months/year back, and the quality had dropped off somewhat, if you can get early ones they are great, as these can be used to just stick a poster up for a while and put it away when not needed. I use them all over the house....sometimes
like in this pic... it's just stood in front of the bedroom TV.
They are great for handling, and temporary display of posters. I have about 40 of these for some of the better/more expensive posters.
The down side is as Tob says, space if you want to store lots, but worth having for the select few posters.