There are no forum rules as to how things should or should not be posted thats down to you and your buyer.
However, I normally add £10 on to my price that I want to cover the costs of sending markers royal mail special delivery insured upto £500.
You must remember untill the gun reaches your buyer you are responsible. So if you cheap out don't get it insured and send it second class, then the gun goes missing, you will have to refund the payment and take the hit for not getting it insured, this is the law.
At the end of the day, a tenner aint a huge ammount and it means you've covered your ass.