Well, unless you increase the cost of the magic item, then you will start running into some balance issues.
Also, remeber that it is possible to simply have magic items that use the quickened version of the spell.
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On a side note, could you please start using the naming convention that most of us on the boards use concerning AoO's?
Specifically, most of us say provoke an attack of oppertunity, not invoke. Invoking implies that you are somehow bringing forth the action yourself, not simply allow the other character the option to do so.