>>If you use an Android device there's a good chance the salespeople already will have a device- since Android smartphones now have achieved >50% of smartphones. Then all you need is the app. I'm not sure of the situation in Aruba but if salespeople don't have smartphones or if the client wants to control the device, you can get Android devices out of China for <$100. This would be a 3G phone into which a sim card can be inserted for data connectivity or texting or one of the new "touch" features that allows phones to be touched to swap contact information. There are plenty of plugins and services to handle payments securely.
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>Yes, Android is very common here, the issue probably would be the printing of the tickets. Those Windows CE devices look very old fashioned, but they have a printer in it which is a great advantage in this situation.
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>What would be the easiest tool to write an app for Android?

>>but they have a printer in it which is a great advantage in this situation.

Check out blue tooth printers, both for the android and for Win CE.