You cannot create customer if you have not a contact. You first need to create a contact as company and after that create contact as customer and/or vendor. On this way, you have link between the same customer and vendor as contact. When you want to add contact person from some company, you need to create new contact as person type and connect it with contact as company type. This is the creating process. When you change some details on e.g. customer, system automatic change this on contact and vendor card.
If you want to use another sell-to address, you need to create new customer with this address and add this customer as customer for invoice.
