I refuse to use royal mail, they lost a laptop i sold and then when i submitted a claims form they lost that aswell!, i wrote them a snotty letter and never heard back from them, so i assume they dont want my custom any more
myHermes or CityLink all the way, not only is it cheaper (i sent a swingarm, carb and shock absorber for about £7, RM would have wanted twice that) but much less hastle, just put your item in a box, stick a label on it and wait for the courier to pick it up, its all tracked aswell so if your buyer says "oh i havent recieved it yet" you can log in online, enter the tracking number and say "well it says its been delivered at 09:15 this morning... check again"