While in concept, this sounds great, it's a scam and they keep your money and do not give you credit where credit is due for shopping trips, and to make it worse their help is nonexistent.
STAY FAR AWAY -
Tip for consumers: If you do not fully collect a certain amount, then they will not let you withdraw, they never let you withdraw because they never give you credit for your shopping trips. So if people send you donations, they are lost to FlipGive they keep the money! stay far far far away.