Odd Trade Issue: Envelope Delivered With Tear. Cards Not Present.

I have a bit of an odd case I would like the community's opinion on: In a recent trade, I received an plain white envelope that had no cards inside. The short edge of the envelope on the stamp side had been slit neatly and to me it looks as though the cards had fallen out. The envelope feels flimsy enough to me that this might be the case and it was not reinforced with tape or anything. The envelope did contain some standard size sheets of paper with a short note on it that did survive the trip.

My question is, how do people think we should proceed to complete this trade fairly? Assuming the sender did send their cards in good faith and the envelope did make it do me (so tracking would not have helped here), I feel that, in some ways, they have executed the trade fully and should not be penalized. On the other hand, the cards did not make it and I feel that they could have been more securely packed. We're talking about roughly $30 in trade value here so it's not a huge loss either way.

Anybody have any good ideas?

Thanks!