= Fixed Quantity
This extension changes the input field for line_item quantity into a select, with options that are multiples of a product's base_quantity. For example, our client sells champaign online, for which most bottles can only be bought per 6. Therefore, people can only buy 6 or 12 or 18 or ... bottles at a time.
You get an extra field 'base quantity' on the admin product form. The options that a client can choose are the first 10 multiples of this value. For example 6,12,18,... 60.
You could change this in app/views/products/_product_cart_form_with_quantity_dropdown.html.erb and app/views/orders/_cart_item_quantity.html.erb (change the '10' or, ideally, make it configurable, and send a pull request)
Copy to your project's vendor/extensions dir, and make sure the naming is right (remove the 'spree-' prefix). Run rake db:migrate. Reboot the server.