How can i disable one day shipping for products i ship FBM?

I recently started selling products FBM. Today a customer bought a product and paid an extra $12~ for one day shipping. When i click “Buy shipping”, it wants me to spend $45 for UPS Next Day. This deeply cut into my profit on the product, and I cannot figure out what the solution is. Why would Amazon charge the customer only $12, then me $45? Any advice/tips would be appreciated.