This is a tricky one. From the OP there are kind of conflicting issues. I will bold and give my interpretation of each section below.
MY STORY: I paid Coinabul 81.8251 btc for an order of silver on July 30, 2012. I paid extra for insured shipping. I didn't receive anything. I have been given no proof that anything was sent. They strung me along for seven months, with half promises of paying out of their own pockets to cover it but never committing to anything. They only gave a final answer when I started a thread here on bitcointalk. Their answer: Sorry, we can't help you.
THEIR STORY: They claim that they sent my order on August 22. They claim that they filed a claim with their insurance company for a lost shipment but the claim was rejected because the value of the order had changed past some arbitrary threshold such that the policy no longer covers it. Coinabul refuse to replace the order or the bitcoins.
Ok, so the buyer purchased insurance from the postal service through Coinabul. Coinabul charges extra to insure the package with the postal service (assuming he did so). The insurance was properly filed, but due to a time lapse limitation the postal service refused to pay the insurance claim.
Assuming all of that is correct, I don't technically think its Coinabul's fault due to the fact that he fulfilled his obligation to ship the package, and to facilitate the proper insurance. It sounds like both of you were boned by a clause in the insurance terms.
But! If that invalidation of the insurance due to price fluctuation was due to an unreasonable time lapse before Coinabul filed the claim, then I would say it is his mistake that should be fixed. Then you would also have to define unreasonable amount of time, but I guess more information is needed, as there are a lot of variables that decide is it Coinabul's fault, or the shipping company's. You can't expect Coinabul to financially burden himself over something that he did to the best of his ability that suits the needs that he had to fill, unless it was an avoidable oversight that caused the situation.