Thanks to smart marketing and big deals, Amazon.com Inc. (AMZN) has become known as the place to save big bucks on everything from housewares to apparel. But consumers may not always get the best bargain and could be making some costly mistakes when it comes to shopping on the ecommerce site.

That’s according to a new Money.ish report that lays out the mistakes consumers make, starting with the mindset that they can get pretty much everything on Amazon for cheap. In this world of instant gratification, Amazon Prime gives consumers the closest to that when shopping online. After all, most products can arrive within two days, but that doesn’t always mean they come with the lowest price tag.

Money.ish, citing CouponSherpa.com savings expert Kendal Perez, reported consumers looking to save a few bucks should avoid purchasing car tires, home goods, cleaning products, clothes and even shoes on the site. Those items can often be found cheaper at discount retailers like TJMaxx and HomeGoods and competitors such as Wal-Mart Stores Inc. (WMT). A lot of traditional retailers also offer coupons that further lowers the price. Other items shopping experts say to avoid on Amazon.com: toliet paper, paper towels, printer paper and power tools. (See also: Is Wal-Mart the Next Amazon?)

Instant Gratification Costs

Another mistake shoppers are making is the need for that near-instant gratification. According to the report, if consumers don’t need to get their order in two days' time, they can save some bucks. That’s because Amazon often gives a discount or rewards points toward future purchases if the consumer chooses non-rush shopping. It make take as long as six business days to arrive, but if there isn’t an immediate need it means more money in the shopper’s pockets.

When it comes to purchasing big-ticket items, timing is everything and that is apparently true of shopping at Amazon.com as well. Based on an analysis by Profitero, Money.ish reported prices change a lot on the website—an average of nearly five times a month, so impulse shopping could cost more than waiting it out. Using a price checking tool can alert shoppers to when the price is low or high on Amazon so they can time purchases to save money.

Prime Doesn’t Always Save

Amazon Prime has undoubtedly revolutionized the way we shop online. After all, spend $99 a year and you not only get two-day free shipping but rewards perks, discounts during special events and access to streaming music and movies. (See also: Amazon's Prime Day Too Big for 24 Hours, Stock Rally Expected.)

But those perks also end up costing consumers because they tend to spend more once they become Prime members. Consumer Intelligence Research Partners pegs the increase at more than double what non-Prime members spend. That’s partly because lots of consumers think that since they are members of the program they need to shop exclusively at Amazon to get their money’s worth. Unbeknownst to them, they actually end up spending more than if they ignored the program altogether.

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