Do not offer a bribe, reward, discount, or deal for reviews. It’s will backfire and get you fewer reviews.
If a customer is happy, they’ll tell their friends about you because you made them happy. If you bring cash into the relationship, the whole thing gets weird:
I love this thing. I was going to tell Shirley about it. But if she finds out that I get one for free if she buys one — then she’s not going to trust me. But I do like it, and she’ll love it. Oh, heck, this all just got uncomfortable. I’ll just say nothing.
Bottom line: The incentive stops word of mouth from your fans.
And, if you can’t make a customer happy enough to review you, then no bribe is going to make it happen. You’re more likely to get slammed.