When I calculate combined shipping, I'll add in the price of the item I'm selling.
For example, if an items weighs 4 oz, what will it cost to add an extra 4 oz to a package? That becomes my 'with another item' rate.
In 90% of the time, my shipping rates are spot on. The only time they are slightly off is when someone purchases two items that total over 13oz as that pushes the shipping into the Priority mail category . But its only if 2 items are purchased. If more than 2 total over 13 oz my shipping rates are fine.