It used to be not easy, no idea if Mendix as great solutions, but the general rule for communicating with the card machine is to treat is as any other application. You need to decide on how to interface with it (one at the time or batch, REST or SOAP, what max delay is allowed),and get that working 100% of the time. So when a user paid for an item, the card machine will send a message to your Mendix app which will get processed. Likely this message needs to pass firewalls and middleware, all of which need to be aware and adjusted. The good thing is: Mendix is only preparing the card machine beforehand usually with pricing data and tracking what has happened afterwards, but is not part of the actual transaction. The card machine deals with that without needing Mendix around.
If you have any say in the choice of the supplier of the card machine, making sure it can communicate with Mendix is a big plus.
Printing a barcode might also be a more difficult issue than it would seem at first glance. Used to have to pre-send barcode-images to the printer but I am hoping printer manufacturers may have improved it by now. Since there is an app for this: https://appstore.home.mendix.com/link/app/7884/ I guess it is no longer a big issue.