Hmm well thats quite a bit more complicated than I anticipated but we should be able to help you get started. Few things to remember is that not very many members have used SambaPOS as a Spa and posted their configurations on the forum. I am sure many people use it this way they just have not been active on the forum. Because of the facts just mentioned it will be hard for me to give you real examples for your exact use however I can give you some tutorials and links that you should look at and practice that will give you knowledge to help you build your own customization.
First you need to create a test database separate from your main production one and use this database to install various advanced tutorials until you can get a grasp of what the features do and how to use them.
Here are some great tutorials you can do that will give you deep knowledge with Entity Screens, Rules, Actions, Scripts, and advanced features. Remember most of these you probably will never use but by building them you will gain some great knowledge that should help you customize your own system.