Which distributor account an order is billed to

The account used depends on how you send the item: drop ship or ship to store. Each distributor's settings page (Distributors, Configure) has a place for each login.

Distributor	Drop ship uses	Ship to store uses
RSR	RSR Dropship Account Number and Password	The RSR ship to store Account Number and Password
Lipsey's	Dropship Email and Dropship Password	Dealer Email and Dealer Password
Zanders	Dropship Order login for accessories, Gun Order login for firearms	Main Account Order Username and API Key
Kinsey's	Dropship Account Number and Api Key	Dealer Account Number and Api Key
Crow	Dropship Security Code	Security Code
Davidson's	Does not drop ship, so every order is ship to store on your account

For Lipsey's, if only one Email and Password are filled in, both kinds of order use that login.

See also Drop ship or ship to store, Which RSR password do you need? and Connect your distributors: what each one needs.