Freelancing Website - Testing & Debugging + Modifications

I am the owner of a freelancing website which has been launched recently but requires Testing, Debugging and Modifications.

You must have a vast experience in Freelancing Websites.
The chosen coder must check all website functionality including payments and suggest modifications required to improve the website frontend and backend. I will give the url only to some selected coders.

I will not accept hourly rates, I want a flat rate please

Thanks