By Hulkster: "would someone get in trouble if they had an international package delivered to an address located on a registered Indian Reservation? "
The short answer is "Yes." States have criminal jurisdiction over offenses committed by or against Indians on the Indian reservation. The Federal government has ALWAYS had criminal jurisdiction over the Indian lands.
Indian lands are held in trust by the United States for the benefit of the Indian people. To illustrate, as a precondition to statehood a new state (e.g., Montana) was required to include in its state constitution that all Indian lands within its borders "shall remain under the absolute jurisdiction and control of the congress of the United States."
States were later conferred civil and criminal jurisdiction by Public Law 280, enacted by the U.S. Congress. Congress enacted Public Law 280 to deal with the problem of lawlessness on certain Indian reservations and the absence of adequate tribal institutions for law enforcement. Also of concern was the lack of adequate Indian forums for resolving private legal disputes between reservation Indians and between Indians and other private citizens.
However, states are not allowed to tax the Indian lands within their borders. Also, a state's regulatory laws (ex: licensing and permits) have no control over Indians within the reservation. States cannot tax income that is derived from activities within the reservation. (So, if an Indian lives on the reservation but works outside the reservation the state can tax the income earned outside the reservation. But the state cannot tax the Indian for income earned within the reservation.)
Basically, it is up to the U.S. Congress to determine what, if any, control the states have over the Indian reservations within their borders. The Indian sovereignty doctrine basically says that the Indian reservations are free from STATE jurisdiction and control. However, the Indian reservations are subject to the legislation of the United States Congress and its administrative regulations. So, the Indians do not answer to the individual states but they answer to the federal government. And if the federal government determines that states should have control over the Indian reservations, then Congress can legislate as such. And they did just that with the passage of Public Law 280.