Always roll them - never fold them.
Sounds like you want one size box to fill all your needs. Get several efficient sized boxes to ship the various sizes you make. That should bring you shipping cost down.
Also - depending on the carrier you use - you should start asking for discounts based on your volume. You do not have to pass on the discount to your customer - just more revenue for you.
Unline and others offer boxes that slide into themselves becoming shorter to suit specific length needs.