By default, Mailspring Pro subscriptions are billed monthly. However, if your organization prefers annual billing, an annual plan is also available at a 11% discount! ($85 USD / year)
To switch to an annual subscription, log in to the billing dashboard at https://id.getmailspring.com/ with your Mailspring ID. Choose the "Change Your Subscription" option and then click the annual plan. Your account will be billed $85 less any credit remaining from your previous monthly payment and a printable invoice will appear in the dashboard for your records.