What are some good platforms, for developer to make payment to designer?

Hi,

I have engaged an app icon designer, whom I know her from Internet.

Is there any platform suitable for developer to make payment to designer?

• So that she know I am able to make full payment to her, once the design work is handed over.
• So that I know I will get the final design work, once I know I make the full payment to her?

Thanks.