x items per bundle / howto

-- HikaShop version -- : 2.6.4 business
-- Joomla version -- : 365
-- PHP version -- : 7014
-- Browser(s) name and version -- : chrome
-- Error-message(debug-mod must be tuned on) -- : none

I am building a shop for one of my customers. They sell boxes and packaging materials online. The more a customer orders, the lower the item price. Some boxes come in a package of 20 pieces, others in a package of say 10. We only want the customers to order complete packages. So if the minimum quantity on a product is 10 (one bundle) and I click the plus sign, I want the quantity increased to 20 and not tot 11.

I've looked around on the forum but found no solution? Is there a workaround or something simple that I am overlooking?