Do your letters even mention a domain or a dollar amount?
 

or do they just say that you'reliable to Acacia for patent infringement?

I dont understand how they can send a letter saying you're violating their patent and not specifically state HOW you are violating their patent.

they expect you to just write a blind check to them or something?

They purchased a database of webmaster information from SOME company. Of course the company that did it will never admin it, we all know it happened and have pretty fucking good idea who.

I think they sent this packet out to everyone in that database hoping that some will bite.

Just like a spammer.. they buy the email lists and send out mailers in hopes that some people will bite.