The general assumption in NOT combining is you are shipping separate packages, one for each item,....OR the cost for shipping is truely additive for each item.....OR you are Invoicing for correct S&H with something in the description to detail what you are doing.
When you have combined, it usually follows that you have special shipping rules or promotions, S&H incremented either by weight or a flat add amount.
It can be difficult to do combined with a variety of items and weights. My "classic" response on combined is....you probably wouldn't want to ship a Tiffany Lamp and a Bowling Ball in the same package.

Danno