I charge £2.50 for first class recorded, actula postage cost is about £2.37. I round it up to cover a new mailer, card stiffners and labels etc. £1.50 for first class and £7.50 recorded. If it is over £15 it goes recorded and ober £40 special delivery as you are not covered for the cost if anything goes wrong.
i ask for F&F if they know and trust me but there is no pressure to pay that way. I wouldn't pay F&F if somewhere not sure about or I don't know the buyer/seller's reputation.