EarlyParrot allows you to stack up rewards encouraging your referrers to get more referrers to qualify for more expensive, limited or desired rewards. Setting up rewards is very easy and is covered in this article
To stack rewards you should keep adding rewards for the same metric and set a higher target. For instance:
Rewards #1: Free Stickers, Metric: Referrals, Target: 2
Rewards #2: Free T-Shirt, Metric: Referrals, Target: 5
Rewards #3: Free 1-year membership, Metric: Referrals, Target: 10
EarlyParrot will monitor all your subscribers and referrers and once a referrer reaches a particular target, they will be automatically sent out the associated reward.