Customer says delivery missing

I had a customer reach out asking when their package will be delivered. Tracking shows it was delivered yesterday and that is what I responded with. They responded later stating “the delivery message says they handed it to a resident-but no one was home”

I’ve not had this happen before. Honestly, after being undercut so hard on my products I’m practically out of business. Replacing products that were lost or damaged by no fault of my own really puts another nail in my businesses coffin. I don’t want to be a jerk and tell the customer it’s not my problem, but shouldn’t this be the mail carriers problem? Before I got undercut I would’ve shipped out a replacement no problem, but I went from making a couple bucks profit per item now I’m making pennies by selling 2 items together.