Ok I think your right I don't really need a domain For each computer that I have.
I think we're not understanding something here. A "DOMAIN" is something that you already have, you configured DNS and ran DCPROMO, and so basically the DOMAIN is present. Now all you are doing is adding to it by joining computers to it, making them "DOMAIN COMPUTERS" and adding users to it, making them "DOMAIN USERS". So yes each client computer will need to join the DOMAIN and each user that actually logs onto one of those client computer will also need to become a domain member, for this DOMAIN model to function properly.
If the computer are logged in on a local account it will work for the POS software.
No it probably will not, that's the idea of the DOMAIN model and using DOMAIN users and computers.
And if I am the only person that work with the server I don't need more domain accounts,
This is not accurate, since the DataBase and other pieces of the POS software reside on and will be hosted by the server you
will need more domain accounts, but not to log on to the server. To log onto the client computers with DOMAIN usernames and passwords, that will permit them to access the software share "WINDWARD"
on the server.
would the domain administrator account suffice? Am I getting this or am I completely lost?.
Generally it is not considered best practices to use the Domain administrator's account to do normal day to day work, nor is it considered a best practice to use it on client machines throughout the domain. So no, don't use the domain administrators account for anything except those tasks that require it but definitely not for normal business activity. It seems that you may be having a bit of a problem, with the domain concept and that's not at all unusual it's probably not all that easy to understand.
But if you think that it is going to cause an issue down the road and make things more difficult instead of easier then you should probably abandon the domain model and go back to the workgroup model before it's too late.