How to pay via invoice on a Public account?
It should be emphasized that all regular (StartUp/Developer/Marketing) accounts are operated under Terms of Service and intended for use by small to middle-sized businesses.
Such accounts are paid via Credit Card / PayPal and don't involve any paperwork including invoices and contract.
Paper invoices are allowed via Private offerings subscriptions. Please Contact Us to get additional info about the Private offering package. Additionally, here you find more information about the Public account types - https://help.pushwoosh.com/hc/en-us/articles/360000480406-Public-account-types