a tool like this might help : https://github.com/wanasit/chrono

the app seems to not redirect to the invoice when it's done taking input

This logic is well thought out! However as you can see it gets complex, very very quickly.

Maybe take a look at a finite state machine, it's a great skill to learn in computing and it's the perfect algorithm to use here!

https://www.smashingmagazine.com/2018/01/rise-state-machines/

I think this works fine in development but won't work in production (at least not with stability), you can't be guaranteed that INVOICE will have the data you need (e.g. if more than one server is running your code the other server INVOICE will be blank) also what will you do if more than one user is using the app.

I would change this so the client app, web app, stores the settings and sends them all to this endpoint via a GET request and query params e.g. invoice?a=1,b=2 etc...

make the chat box take exactly one input every time for example if i'm about to enter " i want to make an invoice" stop taking other input till the program respond

to make the pdf look nice

if you type inside the box something like <h1>hello</h1> the size of the word will change when it's uploaded.