I don't think in SFI you can 'bribe' anybody with anything.
They are in SFI for their own reason, whether it is to make money or have a hobby or a work from home business.
You can reward then when they have done well or you can have a competition.
to use 'Bribing' you need to give them much more than just a $12 gift.
Have fun with your down line in the line of competitions.
Hope this helps.