If she won't approve it, then you might have to return it.
Alternatively, you could tell her any number of items that it is, but she might look it up on google and ask to see it. If this happens, then she might become suspicious that you lied. But either way, try not to worry too much because what's the worst that will happen? It's not like you will be sent to the psych ward. If she makes you return it, then you will just have to find a new method.
Problem solved! I told her that I bought a iPad and she said nice
Oh good God, lol. We overestimated your mom's detective skills.