How to pay via invoice on a regular (non-Enterprise) 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 for Enterprise subscription plan customers exclusively. Please Contact Us to get additional info about the Enterprise package. Additionally, here you find more information about the Public account types - https://help.pushwoosh.com/hc/en-us/articles/360000480406-Public-account-types