I've never posted, but i've recieved 30+ fish by post. They have to be posted in a poly box, usually with heat packs (although i have no idea were you get those), each individually double bagged, but it'd have to be BIG bags for plecos their size, and some people post with "bag buddies" which are little tablets that turn the water blue and help reduce stress, increase oxygen etc. They're only cheap on the internet, and if you did decide you felt comfortable posting, I'd cover the cost of those too as an extra safe measure.
Just enough water in the bag for the fish, with LOTS of air grabbed into th bag (don't breathe into it to fill it with air as this is putting mostly exhaled CO2 into the bag!).
It depends if you feel comfortable doing it though, if you don't I fully understand.
Oh, and postage method depends on the poster, whether it's royal mail 1st class, or guaranteed next day signed for, or via a different 24 hour courier. I've had them delivered by all different methods happily, but if you send by royal mail i'd go for guaranteed next day, which is a bit more expensive but worth it, as first class is taking 2+ days lately!
erm... and if you do post, write "LIVE FISH" in huge letters on every side of the box, with big "THIS WAY UP" arrows. I've only ever had 1 lot of posted fish die, and they arrived smothered in white spot with no "LIVE" marking anywhere on the parcel.
maybe pop into world of water over there but i think that may be pushing it abit 
