I want a Liv doll for my birthday help !!!?

Okay,So my mum says I have too many dolls a whole medium size suitcase full of them.I already have two liv dolls Sophie & Alexis. I want and need Schools out Daniella & Katie.I think my mum said I already have too many dolls and I keep on saying it will be my last doll.How shall I convince my parents to buy me two liv dolls ? By the way they are on offer very cheap from the toy store and a great bargain (�7.49) ! Its going to be my 12th Birthday so... Any ideas anyone ? Thanks x