Have you waited past the Friday after the day you purchased something using FLP?
For the $20 $5 promo, check your email inboxes for the redeem email, probably titled “Your purchase of X qualifies for the Rebate Promo”. If there is no email, create a support ticket to ask for help. Note that the credits, if you have them, expires in 7 days, so check your wallet credits Gameflip | Buy & Sell Games, Gift Cards & More - Gameflip