The POS Change Process
If you find the need to add a custom feature at the POS, you may find a few hurdles in your way. The software may no longer be supported, your requests may be competing for priority with those of a multi-national mega-corp, and it's a process that must be repeated for every change and every POS type within your retail group.
There's also project risk - sometimes the new features are implemented poorly so that they are unreliable or unpleasant to use - and innovation risk where the initiative being pursued fails to achieve its goals, possibly just due to release timing. Then there are commercial issues where the POS provider may be unwilling to provide a feature at all such as access to an external loyalty program, because they want to promote their own program. This 'walled garden' approach is remarkably common.
But if you still want to pursue integration, here is the general process.