FBM Shipping Costs

So, I sell plants on Etsy and now I’m trying to move over and sell them on Amazon as well. My shipping costs very widely depending on the box that I use and how much it weighs but mostly where I’m mailing it. If the plant has grown a little bigger I have to use a completely different box. Sometimes plants have more soil and the weight will be different than the last time. Anything going to the west coast cost more than the east coast. It’s very individualized where the cost can range anywhere from $6 up to $14. I can estimate weight and box dimensions, usually, but the biggest determining factor is location. If I charge everybody a flat $14 for shipping that’s going to discourage a lot of customers for making a purchase, especially in the Midwest and the Southern States. But if I don’t charge enough then it’s going to be like I’m working in California for free. Charging per pound isn’t helpful because everything I sell is less than a pound and a half, and most things I sell are less than a pound. What are my options here?