You'll be charged overage if you exceed your plan limit as an average of the previous 2 months. Overage is billed one month in arrears. You can find out more in this Help Centre article and in our Terms of Service.
If you're on a Basic plan and exceed your 50 invoice limit over a 3-month average you'll be upgraded to a Standard plan automatically. We'll email you to inform you of this. If you're on a Standard plan or above and significantly exceed your invoice limit, we will reach out to inform you as it will likely be more economical for you to upgrade.
We recommend you choose a plan according to the average number of invoices you think you will have to chase each month. You can view the number of invoices you chase each month in-app by navigating to Organisation Settings > Subscription.