Not long ago there was a thread on documentation and a few people speculated that extended documentation (more than one year) would be available. It is not.
I just got my letter and sent it back. $26 for one year.
There was a third party vendor that was apparently offering a service that would document for two years. Since the USCG won't do that, it would be up to the vendor to send in a second documentation to the USGC a year later. I don't think that is likely.
Actually, there are a couple of 3rd party doc services that will handle the document renewal for up to 5 years for only $75 a year. Or you can do it yourself each year with the CG for $26 a year.
I think I'll do it myself.
I got a letter from a third party documentation service a few weeks in advance of my USCG renewal notice. I ignored it.