If the person sent the letter to multiple offices (presumably some democrats) they are guaranteeing that the story gets out. If the story gets out, they are all but guaranteeing that Romney will not pay the blackmail money. He has the ability to claim the documents are faked if he does not pay, but he would basically be admitting guilt if he does.
If the blackmailer is looking for money, they chose about the worst way to go about doing it. If however, they want to call attention to Romney's taxes, they chose a great way of doing it.
However, if their goal is to draw attention to the taxes, it would probably be better to just release the documents out right. While I could see going through the blackmail rouse to draw even further attention, it risks cementing in people's minds that the blackmailer is a criminal and not a reliable source of information.
If you're goal is to damage Romney, not releasing the documents immediately only really makes sense if you don't have them. So, while the schadenfreude in me wishes this were true, I seriously doubt it.