Customers will get "509 Bandwidth Limit Exceeded" error like following screenshot and hard to access your site.
As we're showing in price page, each plan has storage and monthly bandwidth limit. If traffic exceeds its monthly bandwidth or storage does limit, server throttles the accesses.
We're notifying each customers about their site is about to limit bandwidth.
Check current bandwidth and storage usage on Shifter Dashboard, then upgrade a plan to resolve it.
The higher plan has the larger data transfer bandwidth and storage.
Please check support document: Upgrade your Shifter Plan to upgrade a plan.