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 and we would be able to send them over.