You need to read their actual guarantee (in the screen shot below--they've designed the site so you can't copy and paste anything useful). Basically, they don't guarantee a grade at all, and are hoping to sell you their "marking service" after you've created your own essay based on the work you buy from them. It's all a pack of almost-lies meant to get you to buy their product.
Long story short: the company operates a legitimate scam (they won't outright steal your money, they just bury all of the actual details in the fine print so they can deliver you a piece of s-i* and still keep your money).
