If you want to make this solution for yourself and other very technically-savvy users like the HN crowd to use and collaborate on, that's fine. However, despite your declared good intentions, I would never recommend a browser extension like this to non-technical friends and family.
It doesn't matter if the browser extension has an "open source" repo because the maintainer of it can change in the future and then the code is altered to have unethical behavior that doesn't align with the original intentions. Basically a variation of a software dependency "supply-chain" attack. Examples include the xz backdoor and the first uBlock repo being bought by an advertising company (eyeo GmbH). The innocent non-techie browser users will not be sophisticated enough to monitor a Github repo to see if has been compromised and therefore, their "honest" browser plugin no longer does what they think. For those users, it's best to just manually search for coupons with Google/Bing.
Example of how a browser plugin places a "cognitive burden" on non-technical users and just confuses them:
- Chris Aljoudi takes over uBlock and tricks users : https://en.wikipedia.org/wiki/UBlock_Origin#uBlock_Origin
- https://old.reddit.com/r/uBlockOrigin/comments/ija23v/if_you...
- https://old.reddit.com/r/uBlockOrigin/comments/8uxcfv/ublock...