What can we legally do if the mail won’t come to the county approved road that we are in?

I recently moved across town where my family and I bought land and built a new home. We are 1800 feet of the main road and that is at the start of our property line. When we bought this land, we had all intentions of building a nice road (a large portion of our building money) that the school bus, trash and mail man could come down with a cult-a-sac 80' turn around (county regulations) at our gate. We spent a very large amount of money in doing as we planned and in 2 years, the county road engineer said the county would take it over and maintain it and it will be a county road. Until then, we called the post office and had the post master come out to show us where we could put our box temporarily while our road was under construction. He instructed us to place it on the other side of the road which is in a very dangerous spot for us. We are on top of a short hill and no way to pull up to the box so we have to dash across the busy road in order to check our mail. It was not ideal but doable until our road is complete. The County Engineer came out and approved the road. Once the road was complete we put up a very large box (we get large amounts of mail daily) and a sign on the old box post saying it was moved to the new service address. Our mail lady did not bring our mail for about 4 days and then we received a hand written letter asking up to call the post master. My husband did and he told us we had to put our box back on the other road (not even the same road name as ours) that they would not come down our road or a P.O Box. We explained the road was finished and he wanted to come see and did and said the road was very nice but still they will not deliver to our new box. After all the steps we took to ensure that our road would be a county approved road and well maintained it was disappointing to hear this.