The only way you could possibly get that by her is if you purchased it with paypal or a credit/debit card that your mom does not have access to the billing statements, you don't share an email adress, and you would have to get it shipped somewhere other than your own home.
In the end though - you still live under her roof and its only a matter of time before she finds whatever your planning on purchasing. If its something your not supposed to have i suggest just ignoring the urge to purchase it.
well the paypall and debit thing r a problem and i can change my email pass word and i have a plan for the under the same roof thang...
Your 13,your mum should be able to know ,advise on dis-sieving her is irresponsible behaviour if you ask me.
make a new acount using different email adress?