I'm presuming you are keeping the private key for a period of time so that if the note gets lost in the mail, or are charged back before the shipment arrives (or for any number of reasons), you can reclaim the coins. Is this the case?
Yes, for those very reasons.
Once positive feedback has been left, then I'll just shred my copy (I'm a voracious shredder)
Of course, I'll recommend that the recipient transfer the coins off the note onto a more secure address.
what assurance is there that you have a secure system that is not compromised ... e..g., are these generated using an air-gapped system?
None. We're talking about 10-15 coins here, not 100+
Again, it's in the best interest for the recipient to transfer the coins off the note, and I'll make sure they understand this.
And, of course, the model notes in the photos have been specifically destroyed.