What you describe sounds very doable. I would start by creating the POST API call on the R3 site. For this you would define an API endpoint (R3 Builder -> API) with a relation that contains the required attributes (price, sugar). Once you have created the API, R3 shows you previews on how to authenticate and send data via POST inside the Builder.
When the bot calls the R3 API, you could use a trigger function to do the desired calculations and immediately execute the call to the Telegram API. For this we would need to work with the Telegram API documentation to check how a REST call would need to look like (if you have not done this already).
Once the R3 API endpoint, function for your calculation and REST call to Telegram are setup, it should be very easy to extend these for further requirements (similar REST calls, just different arguments).
If you have never done REST calls, it could take you a while - especially with waiting for responses via the R3 forum. If you like, we could build this together via a remote call - it probably only takes a couple of hours until it works. Our company, Lean Softworks, charges little for these remote calls and can work with international clients as well. Just an option, in any case 🙂