For example scenario, let's use a subscription with an annual billing cycle. During the cycle, the customer decides to change from annual billing frequency to monthly. Change is done on 4th March.
|Invoice 1||2019-03-26||2019-03-15||2020-03-15||Purchase fee||3||32.40||97.20|
|Invoice 2||2020-04-26||2020-03-04||2020-04-04||Purchase fee||3||2.70||8.10|
|Invoice 3||2020-05-26||2020-04-04||2020-05-04||Cycle fee||3||2.70||8.10|
Starting from the 4th March, the subscription billing cycle will be monthly and there will be a correction for unused days in a suspended annual cycle. There are 366 days in the billing period and 355 were used. The correction will be for 11 unused days.
In Invoice 2 usage for 1 license per 1 day 32.40/366 = 0.08852. There are 11 unused days in period, correction is 3 * (32.40/366 *355) – 97.20 = -2.92. UnitPrice for month period is 2.70, TotalPrice for purchase fee is 3 * 2.70 = 8.10