>>>most readers of this board think that the seller is MORE responsible for problems with the USPS than the buyer

There is a legal argument to be made here but I'm not going to get into that.

The simple fact is that the seller is almost always responsible for delivery of the item buyer. If you ordered a Dell computer and it never arrived but Dell could show that they had shipped it, would you say just say "ok" and let it go?

I don't think so.

>>>Can someone tell me what I've done wrong in this example?

Certainly.

You are responsible for delivery of the item and thus you assume the shipping risk. The choice to insure the item is yours rather than the buyers.