Build an Online Store

ECOMMERCE: I'm looking to have several types of items that needs to be shipped in a box of 5 based on what the customer selects. Need to have a validation on the cart to see if the items are at 5, if not, do not allow the customer to further go to checkout..let the customer know via dialog box. Here is a link to a site in question that I need to kind of duplicate the function of managing the amount of a product. Steps: 1. Go to this link: [login to view URL] 2. Select any ice cream item 3. Select the + sign to add to cart any of the Mochi Ice Cream. 4. This example, add 2 items to the cart. 5. Add Zip code, next day 6. Select Compute Total 7. Dialog box appears telling the customer to add 3 more items. 8. If you go over (5) items the dialog box will say add more to complete a box of 5 more. It should NOT allow the customer to purchase without validating the complete amount of 5 for each item equaling 5. It needs to be added in increments of “5” 9. Should the customer adds 6,7,8, or 9 items. The system will validate the total amount of items and post to customer you need to add more to cart to equal the amount of 5 per box. 10. The reason is that we will fit 5 items in a box to be shipped. No matter what flavor of item they choose. I need the plugin or eCommerce site to function like this site. NOTE: If you feel we can build a php store front that would be fine. I need to have a CMS to be able to add products. WORDPRESS plugin can be built to do this would be ideal. I found one Woocommerce plugin (ORDER LIMITS) that works very similar to what I need but lacks the ability to allow for customer to choose other items that equals the items. I'm open for suggestions