Dear Employer:
I would like to be considered as a developer for your project. I am an experienced full stack developer with the skills necessary to deliver all of your project requirements (as you outlined).
Here is how I would go about implementing this project:
I would use PHP as the primary backend server-side language. PHP is fast, easy to deploy and scalable. All these traits are important because you want to give your clients the best user experience as possible. Further, if you want to integrate PayPal or Stripe on your website for the payment system, then it would be very easy because both PayPal and Stripe provide fluent APIs for PHP developers.
Sending the notifications is a very easy task. When an event occurs on your website, such as a payment, I can make the website send an email to whoever needs to get that notification. If that's not good enough, I can also build a real-time server using NodeJS and JavaScript that can deliver the message to the user just like Freelancer does (the notification bell on the top of this website).
Finally, for the frontend, I would use the standard frontend languages - HTML, JavaScript and CSS. Of course, I would also make the website responsive (mobile/tablet friendly) so that your users get the best experience as possible.
I am running out of characters, but please feel free to take a look at my profile, and to get in touch with me if you have any questions about my skills or experience.
Thank you for your consideration.