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Amazon only allows users to select 30 items at a time from the dropdown list and add them to the cart. Why not let the users type the number themselves in case they want to buy more than 30? Is there a reason for this number?

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Probably they thought that using a dropdown list instead of an input field would be easier on the user since the user could specify the amount only using their mouse. And they have to stop somewhere and have probably assumed that no one ever will buy more than 30 items. Or maybe there's some other aspect that I can' t think of. –  AndroidHustle Aug 7 '12 at 7:24
It can be that over 30, you're unlikely to use consumer shops (as opposed to retail or how it's called which sells to merchants in large quantities). Also it could be, that they see that purchases over 25 are less than 0.01% of all purchases or similar. –  Aadaam Aug 7 '12 at 11:40
It's likely a space limitation issue with the dropdown, and I assume they researched and found ~99% of people never go over 30 items. In fact I'd bet ~99% of people don't order more than 6 –  Ben Brocka Aug 7 '12 at 12:05
all of the above are part of it. so this simple number becomes part of a slick and easy interface to get dollars in without error messages turning user off buying. user testing with eye tracking was also used to design the ux as best can be. –  Chris Aug 7 '12 at 14:05
The mobile or app version of Amazon provide text quantity for entry. That would lead me to assume that it's not a deliberate restriction on qty, it's a deliberate decision for desktop users. I'd go with @AndroidHustle suggestion - keeping the user flow with the mouse. –  JonW Aug 7 '12 at 18:12

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I can't see that amazon would limit the number of items for a reason as easily avoided as having a long drop down - a simple UI change would fix this. I think the reason is probably more rooted in business logic. I can't comment internationally but in the UK most retailers limit the amount of a single item a user can buy. An old client of mine (hotel chain) when having supply issues tried to buy supplies from a supermarket and were told they couldn't buy the quantity they required.

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Amazon's product selection is too varied for this to be likely; every product line would have different limits. Further, the "edit cart" menu uses a textbox which supports numbers higher than 30. This is almost definitely a UX decision, not a business decision. –  Brian Aug 7 '12 at 17:33
Other products are different on amazon too. Books have a limit of 5 qty on product selection for instance. –  JonW Aug 7 '12 at 18:14
But what about bolts or pens. 30 becomes a low number. Buying 60 pens for your office is hardly a big enough order worthy of wholesale discounting. –  JustinP Dec 4 '12 at 3:25

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