You can find invoices that RoomMate issues to you in the Settings section.
Follow this simple step plan:
go on the Settings
go on the green icon with a white diamond inside, you find it on bottom right of the Settings screen in the Profile Section.
a window will open, here click on Invoicing History
you will find here all the invoices from RoomMate :)
Only the Admin of the account can currently get access to the Invoices