Good app but the usage charges don't change automatically, similar to so many Shopify apps you have to pro-actively change the subscription before any large spike in orders otherwise you're charged way above and beyond the next subscription tier. But you can't always know that, it's impossible to predict the future, perhaps we can see billing structures more often that are not set up to cost you extra money for no reason. So great app but stay on your toes otherwise you'll pay for a year of the app in one month.
Thank you for your feedback. I'm sorry about the unpleasant surprise caused by the usage charge. I'm going to refund the difference to the amount as if you were on the higher plan. I'm also going to implement email notifications to let you know when it's more cost-efficient to switch to a higher plan.