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?
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.