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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
For a payment gateway, you should check out Stripe. You can use their JavaScript SDK which allows you to take payment (https://stripe.com/docs/js). A competent developer with experience in these SDKs could wire up a payment gateway anywhere between a day and a couple of days.
If you want cross platform, check out Quasar (https://quasar.dev), from a single codebase you can build iOS, Android, plus desktop apps for Linux, MacOS, and Windows. If you want browser plugins, you can generate both Chrome and Firefox extensions from a single codebase.