Flip flops are worth a bills, and since they have debonair, that'll add a key, 20 ref will be like 8 keys.
seeing as how a bills is 8 keys unpainted, 9 with pink, green, or team paints, and 10 for black/white. I'd say go with the flip flops
 
Try and sell for something you won't have to re-sell, like keys or metal. I'd try to get between 20 and 21 ref, that way its easy to buy the keys at 2.55, also you have a chance to buy the keys for 2.44 and gain almost an extra ref. Although if you really sit on it for awhile you could get put it for sale at 9 keys or 21 ref (2.33 a key)