For most Ebay sellers this may be stating the obvious. Sending out goods to your buyer by Air Mail up to a Maximun Weight of 2 Kgs, always use USE the Royal Mail SMALL PACKET Service
For PRINTED PAPERS ( books etc ) the Maximum Weight SMALL PACKET SERVICE Allowed is generaly 5 Kgs HOWEVER some Countries have weight restrictions, always check at your Post Office.
See Royal Mail giude PRICING MADE EASY ( dated April 2005), this should be available at all post offices.
A commoent on Queing at the Post Office,, I ALWAYS USE MY LOCAL SUB-POST OFFICE, you get PERSONAL SERVICE, particlarly if it is run by a Self Employed Postmaster, they value your business.