I see. I assumed you were the software user. My mistake.
It is a question of fact, only a jury or judge can answer the question, and the answer could be different every time you litigate it, which is why I cannot answer you definatively.
So the issue boils down to whether the merchant was negligent or whether the software was faulty. I don't have the answer because it is a question of fact.
But it is probable that the merchant's own negligence would have at least contributed to the problem. In some State's that completely precludes them from recovery. But in a majority of States, the award would be split according to the assignable negligence of each party.
So if $100k was lost, and it was 25% the fault of the merchant, the award would be $75k.
I cannot assign a fault percentage, but it does sound like the merchant had a great deal of control over this, and will end up carrying a large portion of the blame.
Which State is the merchant in? Or was there a forum selection clause in the contract?