I don't think any Amazon portals exist... Amazon's affiliate agreement specifically prohibits affiliates from offering any sort of incentive or rebate for purchases through the affiliate link. All portals work that way, i.e. by "sharing" a portion of their commission with the consumer. Since Amazon explicitly prohibits this practice, you are unlikely to find a portal offering miles/cashback for Amazon.

The only way around this is via gift certificates purchased from a third party (e.g. SuperCertificate, etc.) if you can find a portal to that third party...