I figured it out -- so simple
I was making the whole thing too complicated. Only one box size used, it weighs 7 oz. Each item weighs 18 oz. So in eCart calculations I changed TotalWeight from [Weight]+[Quantity] to add in the box weight: TotalWeight = ([Weight])+[Quantity]) + 7.
So, one item in the box weighs 25oz (18+7), two items weigh 43oz (18+18+7) etc.
Seemed too easy, but it works perfectly. Returns the exact USPS postage that I get from checking it with the USPS website "Postage Calculator."