I submit the eticket for purchase of the FP, the redemption of each credit for a particular flight (i.e., the itinerary showing flight number paid by 1 FP credit - since I book one-ways for max flex), the credit value statement, and the boarding pass(es).
That satisfies my accounting weasels.
(They used to also demand the credit card statement corresponding to the purchase until I argued that there is no way to prove I paid the credit card unless I give my bank account statement, and no way to prove I had the money in the bank without taking out another loan, etc.)
BTW, the eticket for purchase of the FP and the credit value statement both show GST/HST as a separate line.