When you offer rewards in a campaign you have the option of assigning each reward its 'Fair Market Value' (FMV). This is the amount that it costs you to fulfill the reward.
So, for things like online shout-outs, video thank-you's, or (mostly) anything distributed digitally, the fair market value of the reward is $0. The FMV is $0 because it doesn't cost you anything to obtain, manufacture, or deliver these types rewards.
However, when you offer an experience, OR physical rewards that you need to ship or acquire, the fair market value comes into play.
This is because the fair market value must be taken into account against the donor's total contribution in order to derive the amount that's tax deductible:
[Donation amount] - [Fair market value] = [Tax deductible amount]
The "Tax deductible amount" is calculated automatically and the donor's tax receipt email (generated by WeDidIt) will include both the total donation amount and the amount that's tax deductible.