This product is purchased based on minimums and units.
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The Min is set to 6 and the units is set to 3
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Quantity Minimums and Units are designed to more or less force the customer to make purchases of a Minimum Quantity ... and if need be, in Units.
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This product can only be purchased if you buy at least 6 ... and after that in units of 3 ... 9, 12, 15, 18, etc.
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If you do not purchase it in the right Quantity, you will not be able to checkout.
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When adding to the cart, the quantity box on the product_info page is "smart". It will adjust itself based on what is in the cart.
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The Add to Cart buttons are also smart on New Products and Product Listing ... these also will adjust what is added to the cart.
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For example: If there is 1 in the cart, the next use of Add to Cart will add 5 more to make the Minimum of 6. Add again and 3 more will be added to keep the Units correct.
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Product Quantity Min/Unit Mix is for when a product has attributes.
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If Mix is ON then a mix of attributes options may be used to make up the Quantity Minimum and Units. This means you can mix 1 blue, 3 silver and 2 green to get 6.
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If the Mix is OFF then you may not mix 2 blue, 2 silver and 2 green to get 6.
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This product has the Product Qty Min/Unit Mix set to ON