Service coupon balance is the same as service credit. When you ordered an annual subscription and paid the amount of $25, the money was added to your "Service coupon balance". Your monthly service fee is then automatically deducted from the "service coupon balance" on your monthly billing date. Your monthly billing date is the day-of-month when you signed up for a CameraFTP account (i.e. account creation date).
This way, you can change your service plan at any time without losing un-used service credit.
( Note: if your order date and account creation date are not the same date, then you will be charged for 1 month or 1 year of service fee first; CameraFTP will then automatically apply a partial-month service credit to your account).