You mean they will charge me $0.99 every times i purchased bitcoin from them?
Yes, it is a flat fee you will be charged no matter what.
After that they charge you a 4%.
So if you buy 100$ worth of you are going to pay 4$ (4%).
If you are going to buy 10$ it should be 0.99 (which is bigger than 4% out of 10$ , 40 cents).
But again this is from my past experience so it might be outdated.