Once you reach 100% of your monthly allotment each additional message will be $0.06/message. The sum of the overages will be calculated and charged on the following 1st, 7th, 14th, 28th of the month.
For example, if your messaging cycle is on the 15th of each month (messaging cycle date is the same date as your billing cycle, you check your billing date in Administration > Subscription). For 1/15/2020 to 2/14/2020, if you hit your 100% allotment on 1/25/2020, all messages from that date to 2/1/2020 will be totaled and charged on 2/1/2020. You'll also get more weekly charges if you continue to send messages for additional messages for 2/2/2020 to 2/7/2020 on 2/7/2020 and the following week as well.
Instead of paying the additional message fee, you can always do a prorated upgrade to the next tier in Administration > Subscription > Update plan. If you are currently on a monthly plan, please remember to toggle to "Billed Monthly" to stay on monthly billing.