Dear Sir.
Your job really caught my eyes.
I submit his proposal after read your description very carefully.
I have rich experience in QA of app.
If you are really doubt on how fast I can complete, just give me a chance, I shall do my best to satisfy you.
This will be good opportunity for both of us, you will get expert developer, and I will get great customer.
I'm fully dedicated onto your project, can devote my full time to complete the job on time.
I can deliver high-quality result to you, fast turnaround time.
Thank you for the opportunity to submit my proposal for your project.
Best regards.