How do I setup combined shipping for multiple listings being bought together?

I’m selling magic cards and I’m charging 0.74 cent shipping for eBay standard envelope. I have a business account and I’m using a shipping policy. I’ve already added 5 cents extra per card under that policy but it’s only being applied to duplicate cards in the same listing but not when they add a 2nd card from a different listing using same policy. They get hit with the 0.75 again, I also went to combined shipping and set up a flat rate setting of add 0.05 per item and it’s still not working. Do I have a setting disabled? Am I just an idiot!? Should I be wearing a helmet? Thanks