The feedback doesn't qualify for removal. You should put the buyer on your blocked bidders list if you haven't already.
You can also respond to their comment that they are known to file false claims of non-receipt.
Is there a way for you to ship the cards with tracking or recordable delivery confirmation especially for your expensive cards? I know cards are small and lightweight but it may be worth your sanity to do it even if you still only charge the buyer $3 for shipping.
I would suggest that when the price goes up to a certain point, ship the card with tracking or delivery confirmation to protect yourself against false claims.You could place a card in a box or whatever is needed to make it qualify for tracking or DC.
By the way, the buyer has 18 retractions so who's the loser?
Non paying buyers deserve unpaid item strikes.