This app was mentioned in 4 comments, with an average of 2.00 upvotes
SlickDeals hands down. https://play.google.com/store/apps/details?id=net.slickdeals.android
I've always had alerts for phrases like "dewalt" or "dog food" but when I made one for "baby" as we were having our kid it was awesome. We probably never paid full price for diapers, formula, toys or gifts for other kids. So easy and saved so much money but it also meant buying ahead when things were cheaper which means we rarely ran out of anything.
EDIT: I forgot, I guess I met my wife on reddit so Boost for Reddit is probably the actual life changer.
I actually use SlickDeals and have it alert me via email and notification. That's how I found out about the Splatoon 3 Pack today. I don't know what you use, so here are links to 3 stores:
And here's the SlickDeals website to check it out before downloading