If posting OS then do not give the buyer a choice of postage options. State what the costs will be with registered post and leave it up to the buyer to make that decision of if they want to purchase at that price with that postage cost. It does NOT cost the seller a thing to post the item as the buyer has paid for it so do not offer a choice.

Yep. When buying and selling, it pays (literally) to think about how you can best avoid getting screwed by either not receiving the item you paid for or a buyer making a false claim about the item.

For a $20 sale of a large letter sent with Reg. Post (putting in the exact postage cost in your listing too), it comes to $17.09 after the fees. Could be worse...