If you upgrade to a paid plan, you will need to enter your credit card into Duplie.  We will charge your card each month the price of the plan until you cancel.  To view/enter your billing information login as an administrator and go to Site and Settings.  Then click on Billing Info.  There you can add your credit card information.  Every time we will your card, we will also create an invoice.  There is a Invoices button which will show you your past invoices.  

For larger organizations, we do allow PO/invoice billing.  If you would like to be billed via electronic invoice, we will bill you 12 months at a time.  Contact info@duplie.com to setup yearly invoices or purchase orders.