I’m looking for some advice from experienced eBay sellers

I’m looking for some advice from experienced eBay sellers.
I recently had a large ACEO order worth around $300. Everything was packed into one tracked shipment. I have a photo of the complete order before mailing, and I clearly remember packing all the cards. I also create an individual packing note for each ACEO as I package them.

The buyer has now left a public comment saying they didn’t receive two ACEOs from the order and mentioned they had already contacted me. However, I never received any private message through eBay, which surprised me. In previous situations, buyers always contacted me directly first, and we resolved everything privately.
The two missing cards are worth about $20 each, while the rest of the large order was apparently received.
I’ve also recently cleaned and organized my workspace, and there were no leftover ACEOs anywhere, so I don’t believe they were accidentally left behind.
Has anyone experienced something similar? Could this be a packing mistake, a postal issue, or is there something else I should consider? Any advice would be appreciated.