This looks like a amazing idea. The whole E-commerce transaction done over a Bot. Would love to see this in action.
I think any such use-case would definitely require a way to do basic calculations like the one you mentioned. and even more advanced stuff like giving discounts, adding coupons. But to start with, doing basic calculations within the bot is something we have planned. We will be working on this soon, so that you can write basic math expressions and sove them for billing and scoring things.
Currently the most efficient way to do such calculations is using an external math calculating API like this one: http://api.mathjs.org/
Here you simply send out the expressions you want to calculate and the API will return the answer in the response, which you can then refer to and display it to the users. Check it out, it is very simple and fun to use.