assorted wine crates

you can use this code for any product, not only for wine.
this contribution was built because i needed to sell wine in crates of 12 bottles each. i also allow orders of single crates with 6 bottles, but if customer wants to order more than 6 items, it always has to be in multiples of 12.
additionally, we wanted to be able to allow the ordering of wine crates with assorted wines from specified categories.
my code allows the customer to order an assorted collection of products from a specified category (or subcategory), or a list of categories (or subcategories), recursively including all sub-categories. the total number of ordered products (e.g. wine bottles) from these categories has to be 6 (modifiable) or multiples of 12 (modifiable), otherwise we send a warning message, and redirect customer back to his shopping cart.


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