Is "Pay back my money!" enough natural enough to be used as a website name / company name?
Welcome to the forum.
First, note my corrections above.
"Pay back my money!" is a natural enough statement/demand if you're talking to someone who owes you money. We don't know if it's suitable for a website or company name without knowing what the company does.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.