Here you find  the prices list.

Online-billing-service-com is a subscription-based service. You only pay for how long you use the service. The invoice is issued monthly, or you can choose to pay in advance with the following discounts: You get a 10% discount for 12 months, 7.5% for 6 months and 5% for 3 months. Due to bank charges, the Basic subscription can not be paid monthly.

Can I change the type of the subscription?

Sure. Acces this link for details:  How to change subscription type.