Without a COD that can be directly linked to the seller, it does not matter if it was a week or a year. Even if a buyer thinks there is nothing they did incorrectly (and there may indeed be nothing done incorrectly), that does not exclude the possibility of something having happened in the post-delivery environment (seen, unseen, known, or unknown).
The reason for me is that these are living organisms and living organisms are not perfect and generally not so much immortal. I could, for example, receive a seemingly healthy by all measures parrot and it could have an aneurysm that kills it the next week. The same thing could happen to a tortoise a month after I send it out. We are not dealing with an immutable identity like a hunk of gold, so I generally only guarantee live arrival and the described condition. If the animal outlives the new owner and the owner's kids and then passes into the care of the grandchildren, that is pretty cool. If it dies a month after arrival, then that is simply among the risks we take.
I receive stuff that dies every now and then. I received a group of sulcatas that were five months older than I thought they were (by my incorrect initial assumption) and they had nearly zero growth to them (a generally bad sign). 75% of that group died. This was only a couple of weeks after arrival. I had many, many counterparts from my own breeders and those of other sources living in identical/parallel setups that were thriving. Does that mean the seller did something bad? I could not say so for a fact. Everybody tested clean on multiple occasions. Necropsy revealed nothing of value. Since they arrived alive and in the condition they were shown to me as being in, I just let it be and moved on.
I have never had a puppy die, but puppies die every day. I have never had a human baby die, but human babies die every day. From neonates to "geronates", every species dies and often not when we want them to or are ready for them to. Some fail early, some fail at end of conventionally "expected" lifespan, and plenty in between.
I have a difficult time placing an expectation on something that, as an individual organism in a much larger numerical set, has no obligation to adhere to whatever my expectations might be for survival. Following that, I would have a difficult time trying to obligate a human being to control such, as that is a variable which is often not within our control to begin with. We can do the best we can to nurture the prolonging or manifestation of potential, but the thread may be cut early despite our greatest efforts.
In your specific instance, about 56 days have passed. During those 56 days, you saw no indication of a problem. Unless you have overwhelming evidence, there is nothing to really lay at the feet of the seller. She was normal for nearly two months in your care and then no longer normal in your care in one punctuated swoop. Not in the seller's care. Not upon arrival to your care. As far as I am concerned, you have had an unfortunate experience, but it does not constitute a case and there is not really room for expectation surrounding it. Terms were fulfilled.
