If you’ve checked your AmEx offers lately (and you should to check for no lifetime language offers and $20 off Amazon) you may have noticed that some offers are a percentage back with a cap (e.g get 6% back on up to $60 in purchases). With these offers you also need to use a specific link (joindrop.com/amex/[merchant name]). For those not familiar Drop is a card linked app that offers cash back at specific merchants.
In this case it’s basically working like a shopping portal, but you need to use a specific card and save the offer on amex offers first. Because you need to use a specific link this means you won’t be able to stack another shopping portal with it unfortunately (it does seem that American Express is not paying out when not using this specific drop link). Currently this is only on a few offers that are a % back rather than the standard spend $xx, get $x back offers.