I have multiple products (books) and need to combine shipping cost on checkout. (example: buy one comic book - shipping costs $5.65 - but I can also fit another 20 items from the same Category in the shipping package with no additional shipping cost)
Some items are small and can be fit into the same shipment for a flat rate at no additional cost, up to a certain number of items / volume of the total package sent.
Currently it is "per product" shipping cost. I do not need to bill customers per product for shipping when I can fit multiple items into one shipment for the same price.
I do not wish to purchase any extensions. I need either a free extension that will solve this, or advice on how to write the code in Magento please.