You can earn cashback twice on one purchase: once from your credit card and once from a shopping portal, and the two stack.

Cards pay a fixed percentage automatically at every store, while portals pay a variable, store-specific rate that requires clicking through before checkout.

On a typical order, the portal wins when its rate beats your card's rate, the card wins when the portal rate is low, and stacking wins almost always.

Portals have exclusions for gift cards, some categories, and coupon-stacked orders, and they cannot credit purchases retroactively if you forget.

Install the portal extension, use your best card by default, and check portal rates before big purchases to maximise the combined return.