IMHO, that's no excuse. The seller agreed to knock $10 off, but that doesn't suddenly absolve him from doing a poor packaging job; if that was the case, then he should have told the buyer: "Okay, I'll knock $10 off the shipping, but that means I'm going to half-ass the packing job, too."
If, for instance, the seller chose to use a "cheaper" service b/c they were receiving less cash for shipping costs, the ownness is still on them to make sure everything arrives safely.
Regardless, it's up to the buyer to discuss the damage with the seller and come to an arrangement that suits both parties--if the buyer didn't at least initiate the discussion, then I suppose it's not even worth mentioning in the feedback post.