We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/about-mailspring-pro-subscriptions/304
If you pay for Mailspring Pro, you can view invoices / receipts your monthly payments anytime by signing in to https://id.getmailspring.com/. A list of recent payments appears on the identity dashboard:
To view or print a receipt, click the "View / Print Invoice" button to the right of the payment.