The easiest way to you, and possibly the safest way with minimal possibility of damage would be unmounted in a tube. Most postal tubes are made from quite solid material and will give adequate protection, you can always increase the protection by wrapping or roling the painting in bubble wrap also.
A flat pack, unmounted or even mounted paintings, has more chance of being damaged. One thing, should you choose to flat pack your paintings, is to use solid card on either side of the painting plus bubble wrap. You should then put the whole wrapped painting in a cardboard box and also use corner protectors to help decrease damage if the parcel is dropped.
Make sure you include the cost of postage materials when calculating the postage to the customer - where possible you may also want to consider whether the customer could collect the painting from you.
I hope this is of some use to you - should you require any further help please feel free to call us on 0800 980 1123 and myself or one of my colleagues will be happy to help.
Nadia - SAA Administrator