USA is part of the standard delivery of SAP ERP.
Therefore you normally do not need to install an additional add-on.
However you need to implement the country (e.g. customizing ... etc. ). Please have a look at the country info:
One special requirement in the US is that many customers make use of an external tax calculation system due to the high number of regional taxes.
There is even a book called "US Tax and SAP" dealing with US taxes only.
In many books on SAP ERP FI, I read about CAUS chart of accounts. I'm using EHP6. In my version of SAP, there's no CAUS chart of accounts. I did country installation for US, it didn't produce that chart of accounts for me. I ran the same country installation in EHP4, it didn't created either. I would appreciate if anybody can help me out. I'm following two books on FI & CO. They refer to CAUS.