Any site that money changes hands is no small feat and should be done carefully and heavily tested. Not sure the scope of your project, and I have not worked with TicketNetwork’s Web Services but most things like that have an api in which you interact and they give you sample code that is often everything you need and just plugging in the correct variables etc.
I would assume it goes something like:
Your script POSTS to their web server with the type of transaction + credientials and likely a bunch of other information
Their script response with some set of confirmation presumably documented about whether the transaction was successful or not and why/why not.
Which you then have to handle that by perhaps updating your db, notifiying the user, perhaps some email notifications.
Your code can likely be in any file so long as you have the right credientials and study their API enough to know what to send/receive