Hi Neil,
To purchase services in Microsoft Online Services, you choose one or more subscriptions from the catalog and then pay for the services by using a credit card or by requesting an invoice. Your subscriptions are renewed automatically every year.
For Pay by invoice, you may refer the information on below article.
onlinehelp.microsoft.com/.../gg570309.aspx
If you also have questions, please contact our sales and billing support to get a quick answer.
Contact Support for a billing or subscription issue
http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff652548.aspx
Thanks,
Jason Jiang