It’s simple, really. Look at the search terms that show up for your products. Do some of them seem unrelated? If so, you are bidding too high. You could spend forever trying to add negative keywords in an attempt to prevent google from making you show up for overly broad / unrelated terms. It won’t work. If your bid is high enough Google will find a way to spend your money.
By lowering your bid you drop out of the race for all but the most relevant terms. That’s because someone else who has a higher bid is going to take the spot. Let them, please.
The irony is that you’ll be spending less but you’ll probably make more overall revenue because your budget won’t be wasted on garbage terms. So you’ll pay less AND get only relevant visitors AND get more visitors.
If you want to get fancy with PLAs I would recommend reading this article. We use campaign priority combined with negative keyword lists for low intent searches. It’s complex, but there are agencies that will help you set it up. I used marketersbraintrust.com. They did the setup and taught me how to run it by myself. It was the best agency money I’ve ever spent. It was more “teach me how to fish” than “subscribe to my fish sandwich” service.