Birthday

My mom's birthday is April 28 and I'm 12 years old so I can't really go out and buy gifts without her finding out its for her. I've decided that I'll buy her a gift online so she wont find out until it arrives but I'm jot allowed to use credit cards. my mom lets me buy prepaid credit cards in case of spams, theifs, bad websites, ect. And I know exactly what I'm gonna buy her online an how much I need on the card (50$). Whenever I buy a prepaid card it can max have 25$ so I just need some help on finding a way to get her to let me get a 50$ card AND take me to buy it ...

HELPP!