Are we exploring the SSO option between Configured Commerce and BillTrust for users with access to multiple Bill-To? One possibility would be to send the currently selected Bill-To.
Related support article:
According to the Billtrust SSO integration documentation, Billtrust assumes that a given username can only access a single account (that is Configured Commerce Customer). This could cause an issue if a single Configured Commerce User can access more than one Bill-To Customer.