Do you know how Retailers trying to outsmart us consumers? The answer is: Dynamic Pricing!
It's a common practice for airline tickets etc. (perishable goods) pricing. But more and more online retailers are using this tactic to outsmart consumers. They will utilize the cookies on your computer to analyze your shopping behaviors. One finding is that Chrome users tend to pay less than Firefox users for the same item.
Once the retailers recognize your shopping pattern, they will adjust prices instantly. For example, you clicked an item shown as $20.05 and put it on your shopping cart. But you got distracted, when you came back later to check out, you could find the price jumped to $25.02!
Is it legal? Perfectly so because the T&C's of any online retailer always says "prices are subject to change without notice".
What can a consumer do? One thing, price shop. Always compare prices between different online sites. My rule of thumb in online shopping is always use Amazon as the last check, due to its wide range of product offerings, excellent reputation, and the fact that there are always deals going on (Amazon has a marketplace that many sellers could sell at different prices for the same product).
To make it easier, I bookmark its Today's Hot Deals page, and always go from there. I highly recommend you doing the same by checking it out.