The module adds a new product attribute in the 'General' tab, called 'Multiples Of X' - default set to 1. For example, set it to 5. This forces products to be ordered in quantities of 5, 10, 15. A warning message is shown if it isn't. Shows (+) and (-) buttons next to the quantity box on the product page.
Note: the lowest possible Multiples Of X will act as a Minimal Qty. For example, if your Multiples Of X = 5, but your Minimum Qty = 2, then of course "2" can never be ordered, because "2" is not a multiple of "5". And thus acts like a minimal quantity, forcing you to order at least "5".
In the admin, you can create a new order manually. This extension will enforce the qty multiples there as well, by silently updating the qty to an allowed multiple of X.
adds attribute to General tab on products
enable module per store via config setting
product-specific Multiples Of X setting
this version also enforces qty multiples in the admin when creating an order manually (silently updates the qty to an allowed amount)