Need Help From Mothers!!!?

Hi I'm Olivia, and I'm a Freshman in Highschool. So the other day my mom and I were talking about jewlery and i told her i'm more of a clothes and shoes kind of girl :) and she said "Okay, let me buy you those UGGs first" then i stopped her and said "Noo it's okay" and she asked me why the change of mind and i said "I can't let you spend 200 dollars on me like that, besides i wouldnt have to live with the guilt :)" so yeah. but now i really want them! And i know it's 200 dollars and that sure is ALOT but i am planning on paying her back for them! like i am hoping to get a job over summer and my birthday is in summer which means like bday money and stuff and i mean like i would definitley definitley DEFINITLEY pay her back for them! even if she refuses i'll still pay her back no matter what. Please please please help me! Like what should i say to my mom should i just be like "Hey mom...actually i do want the UGG boots but i will pay you back" or what should i say? Please help me! and it took a month for her to think it over and agree to get me them, but i know that i'd get an automatic answer if i asked her again. So please help!