Billing in Freshpaint is determined based on the number of MTUs (monthly tracked users). In other words, the number of users that Freshpaint has data on. More specifically, the number of MTUs is equal to the number of non-identified users plus the number of identified users.
If a non-identified user visits your site on their phone and on their computer that will count as two MTUs since they have a unique id on both their phone and on their computer:
2 MTUs = 2 non-identified users + 0 identified users.
If you later assign a single identity to the user on both devices, Freshpaint will now recognize the two devices as a single user. This will bring your MTU count down to 1:
1 MTU = 0 non-identified users + 1 identified user (that uses two devices).