If you want to issue a one-time invoice for a certain service that is provided, for example, the whole year, you have to be careful not to confuse the total amount charged with the price of service per month. If a given service costs, for instance, 72 EUR per year and you want to issue a one-time invoice covering the whole period (12 months), do not insert 72 EUR as the price of service. If you did that, the system would do the following: 12 x 72 EUR (which is not what you want). Instead, insert 6 EUR as the price of service (12 x 6 EUR = 72 EUR).