There are always the same two envelopes. Whether one of the envelopes is in my hand or on the moon makes no difference to the probabilities involved. A sequence of choices makes no difference to the probabilities involved - there are always the same two envelopes to choose between. The average value of the contents of the envelopes also has no relevance to the probabilities involved. My choice - no matter how many times I do it - always has a 50% chance x and a 50% chance of 2x