Amazon routinely moves prices up and down and sometimes for short Periods of time. There is a site called camelcamelcamel.com that tracks pricing on Amazon. You can see history of the price and set alerts for price drops. Great way to catch a deal that might only be in play for 30 minutes.
Here is an example. The ear protection I just bought. Price is all over the place but for one day in June it dropped big time.
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