Apply account balance to open invoices

[Rowbotronics]

WildApricot does not give users a way to apply a positive balance towards an invoice. We could probably do this through the API. When we eventually get the automatic charges for the 3D printers up and running it would be very handy to be able to pre-pay and not have to deal with every 32 cent print.

• Check to see who has an open invoice and a positive balance
• Apply the balance to the invoice
• If invoice is fully paid send email to notify user and tell them their remaining balance
• If invoice is not fully paid send an email notifying user of the updated amount and payment options