Maybe I didn't do a good enough job of explaining how the extension works. All data is stored locally. The extension does not ask you to enter any card details, we only ask that you tell us which card you have (Yes or No type of information).
For different card variants, the user can specify while adding the card to help calculate the rewards appropriately.
We detect whether you are on a checkout page locally, and then calculate based rewards based on your cart value, and where you are shopping from. No identifying information leaves the browser.
Re: being fishy, we do not require any login or sharing of any personal information. The extension is approved and published in the Chrome extension store, which involves clearing Google's privacy and safety checks.
Yes, we might be buggy, but fishy is not us. We came here hoping to reach a wider community of users so we can help people manage multiple cards better and yes, also get some feedback and feature requests to improve.