In general, this add-on is decent, but I've had three big issues with it.
1, It's dumb. There has been an occasion where I had manually applied a code and by running Capital One shopping it actually removed my (better) code, and applied one that gave me less savings. Irritating to say the least.
2, The second issue I have encountered is it doesn't seem to be able to apply more than one code even if the codes can work together.
3, It's inconsistent. I removed a code, messed around in the cart, ended up with the same stuff, came back. When Capital One ran again, it gave me a code that had didn't save as much. Why didn't it apply one of the better codes? This is also how I found out about multiple codes, because I added the first one back (I don't trust C1 shopping so I saved the code) and managed to save the most with the combo of codes.

It has definitely located better codes to save more money with minimal effort from me but this is not a perfect solution, at least on the coupon and discount code front. Still easier than me searching them up, though!