T-Mobile U-Fix + Web 'n' Walk help

[Guest Meatball1989]

I'm hopefully getting my first contract phone soon (Vario II) The only way I am able to get a contract is if my mum registers it under her name (since I'm under 16) She's done this before with one of my siblings, but the bills went sky high so she's been put off letting me take out a contract. I recently discovered U-Fix and it seems to be the perfect solution but can you please answer these questions before I talk with my parents.

1) With U-Fix I pay at set amount a month (in my case