Does said death-mail have legal standing?
It would depend on the state and the intent of the mail. Wills have traditionally required a lot of formalities to be effective (e.g., signatures of the testator and witnesses) with some interesting exceptions (e.g., the "holographic" will, a will written *entirely* in the hand of the testator). Many states have loosened up on the formalities though, but the document would still need to be a clear expression of the intent of the deceased.
Plus, there could be some confusion if the document promised one thing, but another document promised something conflicting. Which one takes precedent? Would courts decide based on when the documents were created or when they were intended to be delivered?
Another thing to consider is that these messages to be sent after death would probably be easier to obtain during lawsuit discovery than a will. A will written with an attorney's assistant may be a privileged document. If subpoenaed, The Last Messages Club may (or may not) fight to protect messages not yet delivered from discovery. But courts are less likely to protect such messages from disclosure than a will (although, perhaps various courts will agree to in camera review, meaning that the judge looks at the document first to decide whether it is relevant to the litigation at hand).
One last thing to consider: your message might produce evidence that could lead to a lawsuit against your estate, and thus hold up your assets. The Last Messages Club could mitigate this problem by allowing your messages to be put on hold for a number of years after receiving proof of your death. This could help ensure that your estate has been disbursed and increase the probability of a statute of limitations being hit.
As is, The Last Messages Club should probably be used to send those personal messages that don't have any real legal implications. If you want to write a will, hire a lawyer and do it right, don't try to do it yourself with a somewhat gimicky service.