With a fixed cost of $$100$ per order, Nathan decided it was vital to get his moneys worth. His monthly demand for energy drinks was $10,000$ bottles and a holding cost was estimated at $20\%$ of unit cost. The mail order company offered him a couple of possibilities$-$ he could pay $$4$ per bottle for orders of up to $10,000$ bottles. After that threshold, he would pay only $$3\mathrm{.}98$ per bottle, and if he ordered $20000$ or more bottles in an order, he will pay only $$3\mathrm{.}96$ per bottle. What is the best order quantity for Nathan to use?