On the web, accessibility overlays are common; a bolt-on intended to absolve the developers from having to think about accessibility with the promise of a magical fix. I applaud that you have taken the step to look for professional help to make your app more accessible, but consider what you are asking them to help with - by shifting accessibility left you will get a better experience for your customers, faster, and at less expense than testing for accessibility at the end. An accessibility audit is part of the process of making an accessible app.