To accommodate this I add additional supports which is why the cost is more. For the King size I add 2 more pieces of angle running from the head to footboard. I then supply you with pine boards that you lay across. Your mattress rests on the boards. You are welcome to get your own pine boards and I'll reduce the price by $80. (buy the other King version listed and tell me you want no box spring dims)
The other factor to consider is the alteration of the heights of head and footboard.
I like the idea that the footboard is flush with the mattress. The standard height of the bed is 7" off the ground. If you wish for me to make the footboard flush then please tell me the thickness of your mattress. Adding 7" plus 1" for bedding will tell me the height of the footboard. You may request the headboard to be a height of your choosing. 45" or lower is common.