You cannot prove that the shoes were in the package.

The items were sold with Simple Delivery so I assume Ebay issued two separate labels, one for the shoes and one for the boots.

If you sent both items using one label, then the tracking will not show the shoes as delivered.

Without tracking proving delivery, Ebay will expect you to refund the buyer.

I can see how frustrating this is as you were probably trying to do the right thing by sending both items together, but you have left yourself vulnerable to this sort of claim.

You can report the buyer for abusing the Money Back Guarantee but you won't know if Ebay take any action and you will still be out of pocket for the shoes.