Web chat with payments, account mgmt, transportation and travel APIs search and book features

We are creating a web app with payment gateways (stripe), web chat to be used for customers to enquire us and human operators to fulfill their requests and such requests involve API calls to find flights, trains, hotels. All customer data acquired from the chats and saved by themselves on their profiles need to be accessible for analytics. For some examples of how it should look like and a feel for the complexity see the pics below. For UI design: not needed as we are just using the web app within web views. We will provide logos and colors. The routing of messages of interest shall be done via a centralised notification management system like [login to view URL] or similar (you should propose the right solution here). Backend running on Google App Engine and PostgreSQL running on Google Cloud SQL. Some things we'd like to know from your proposal of are: - Message queueing strategy? - How to provide real time messages on the chat? - Best way to integrate APIs for all tansportsations and accommodations services? e.g. tradeoffs when using single API for everything like expedia vs lufthansa api and emirates API and so on Please, along with a suggestion for the above, provide: - project estimated length, - some pieces of tech needed for sure, - approx. cost - most time consuming part