Each morning the workflow fetches the product pages on your watch list, reads the price from each page and compares it with the price logged the day before in a Google Sheet. The price is read from the first element with the price class, so you can change the selector for other sites.

When a price has changed, a message with the old and new price goes to your Slack channel. Every product is written back to the sheet with the time it was checked, so the log stays current whether or not the price moved.