That's a point, actually.
The strain caused in spellcasting is from the effort needed to take that energy and shape it to a purpose. Otherwise using Sponsored Magic wouldn't cause Mental Stress, because the power is coming from elsewhere, not from yourself.
What makes Enchanted Items useful is that the energy in them is already shaped. The work has been done, the power just hasn't yet been unleashed. That's why they don't cause you to gain Stress.
If this item is just a place to store some power that you use later, it works better, mechaniclly, as a Focus Item, Aspect, or Item of Power. Its use would still cause Stress, however.