Unless of course they have a disclaimer of responsibility like this
From their web page Shipping and Handling
USPS offers speedy and reliable service at a low cost. However, they can and will occasionally make the mistake of mis-routing or losing a package. Unfortunately these errors are out of our control and we are not responsible for errors or delays by USPS.
It's up the the merchants decision of good customer service as to wether they will replace the items. They may be of assistance with helping to track the package. I've never dealt with this company, so I make no claim to know how they would react to a lost package. However, their policy is clearly "we are not responsible." And I've shipped products world wide for over 30 years, that's why there's freight insurance. The vendor is never responsible after it's shipped unless that is stated in the terms of the contract/purchase order. It's the shipping companies responsibility.
The disclaimer means nothing .. as you know, Credit Card transactions are allowed to be disputed if the customer has a problem that is not resolved by the vendor .. like I said, I've sold mail order for 35 years, I know full well how it works .. and I'm guessing you do to with your experience ..
Of course, if the customer paid with cash or a check, the disclaimer may work, but then you run the risk of a report to the Better Business bureau ..
These are all reasons insurance should be purchased from USPS, which, unfortunately, most vendors won't do because it costs money .. so, most will roll the dice figuring that if 1 out of 1000 packages got lost or damaged, they are still ahead money wise even when the order is replaced or refunded since USPS insurance runs $1.65 per $50.00 .. pretty much Mail Order 101 ..