Is it possible to modify a sales transaction on the square app via the API before it results in a sale? I'd like to add an item to a transaction via the API based on certain conditions (eg: particular customers get a free item). It appears the API supports only 1) collecting detail of transactions already completed, OR 2) creation of entire transactions, based on basically creating your own the square app. I was hoping to build something simple with the API, intercepting transactions as they are created by the ipad square app.
