CRA considers that no invoice is required for an on-supply covered by an ETA s.156 election

Where a member of a corporate group makes all the purchases (properly documented with invoices from the 3^rd party suppliers), and then resupplies the purchased goods or services to other group members with which it has made valid ETA s. 156 elections, CRA does not consider that there is a documentary requirement for such resupplies to be evidenced by invoices.

May 2015 CPA Roundtable, GST Q. 6 under ETA s. 169(4).