The orders per month metric used for pricing is a rolling 3-month average in order to address seasonal variability with order volume. It is calculated as an average over the last 3 months across all your sales channels.
For example, if you did:
60 orders the first month
75 orders the second month
160 orders the third month
This would still put you in the 0 -100 orders per month tier since the average for the past 3 months would still be 98.
However, if you were to do another 160 orders the fourth month, this would then put you in the 100 - 199 tier and you would be billed for that the next month.
We monitor these stats automatically so there's nothing you need to do on your part!