How much contact with a state is sufficient for a court to exercise jurisdiction over the person without offending the due process requirements of the US Constitution?

How much contact with a state is sufficient for a court to exercise jurisdiction over the person without offending the due process requirements of the US Constitution? Is there any situation where one could argue a slight amount of contact with the state still justifies exercising jurisdiction? Can the court exercise jurisdiction over a business that regularly ships products to customers in a state but does not have a physical presence in the state and is not registered to do business?

Next, consider this factual scenario: Elena lives in Vermont and has a small business that manufactures a product for pets and sells it to retail establishments throughout the United States. She takes orders on her website and ships her product through a third-party logistics company. Gary, one of her retailers in Montana, is not happy with the quality of her product and demands a refund. When Elena refuses to refund Gary’s money, he sues her in Montana court. The Montana long-arm statute allows for service of process on a civil defendant in any state if the defendant is a business entity and ships any products into the state of Montana. What constitutional argument could Elena make to defend against being served with process and called into court in Montana?