If you signed up before Aug 9, 2021, please click Previous plans to view your applicable plans.
Account administrators (your role on the portal) would be able to download their Freshdesk account invoices from within the helpdesk.
Please navigate to Admin -> Account -> Plans and Billing -> View Invoices from where you could download them.
Kindly note that this is available only for online payments that have a registered card on the portal. For accounts with offline modes of payments (bank transfers and PayPal), contact support@freshdesk.com and we would be able to send them over.