These are for your own records.

You can put some additional information in the Notes, like "seller agreed to include some extra items" or "seller is questionable", etc.
Payment status is the same thing. You can mark the item as GotItem so you know which item was received by you and which one you still expect in the mail, and if it's not marked as GotItem then you should contact the seller about it.

What function of Notes in Details?
There are fields 'Transaction status', 'Payment Status' and 'Personal notes..'
How can I use them?