Archiving an invoice works in the same way as deleting an invoice.

To Archive an Invoice in the Web Portal > CRM > Select the Box next to the Invoice you are wanting to Archive > at the bottom of the page click the Select Action drop-down > Archive > Go.

An invoice cannot be archived in the App it can only be cancelled. Once it is cancelled it can be archived on the Management Portal. Ensure to use the filter on the Management Portal to see your cancelled invoice to then archive them.