This is where Mackinnon's new start-up, Kard, comes in. The New York City-based company, which he founded in December 2015, aims to help consumers maximize their credit card rewards programs by making them aware of the best merchant-specific offers when shopping.
"Our users can earn between 7% and 8% on their annual spend," the CEO tells CNBC. "Of course, it depends on how much you're spending, but that could be a couple hundred dollars to over a thousand dollars in extra cash back, miles, and rewards."
To use Kard, you simply install a free Chrome extension, which you can do on Kard's website, and type in the credit cards you use. You don't give any personal information, just the type of card. Then, whenever you're shopping online, Kard will notify you if any offers are available and help you activate the deal.
"With the extension, you'll get a little push notification in the top right-hand corner of your browser that says something like, 'Use your Amex card and get $25 off your $50 purchase,'" Mackinnon explains. "It's very unobtrusive to the user, but provides a lot of value. If you don't want the offer, you can just X out of the notification."
Here's an example of what a Kard notification would look like on your browser: