18 Jul 2012
24 Sep 2012
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I've actually been pondering this myself recently and came up with two options, as there appears to be no built in way to do this yet.
First, you could modify the both the product template and the shopping cart template so that instead of a textbox for quantity, a dropdown is shown only with the options you want. However, would probably only work for setting a range.
The way I ended up doing this was to simply add an intermediate page between cart and checkout. I linked the cart to my intermediate page, and this page had a widget that handled the logic for checking the cart quantities, and showing the customer a message to change the quantity if needed. If all was correct I would simply redirect to the checkout page.
Both of these have their pros and cons and involve a bit of work, but I hope they give you some ideas that might work for you! And rest assured that I too let the team know about this suggestion for future consideration!
hope this is helpful,