What is popular now is to link into your point of sale system a laptop rather than a desktop. The cost is a bit higher, but it has a heap of advantages, e.g. you do not take over a computer at work, it is easy to move where you need it, and you can easily take your work home with you. I have been to meetings where we all have our laptops in front. It is the in-thing now.
Unfortunately, one problem is that one of my clients went looking for a price for an item, so left his laptop alone and in seconds someone grabbed it and was out the door in seconds. When he came back and wondered where it was? By the time anyone realised it was long gone.
So if you are going to use a laptop in the shop, I would suggest that you get a laptop security cable lock. You can pick one up for about $10. That should stop your average thief looking for something easier to steal. Of course, it will not prevent the guy with the bolt cutters, but he is a much less credible threat.
I have seen some security footage of some thieves hitting a shop after hours, and it shows them pulling the cable trying to get the laptop and giving up, so here the cable worked. Remember these guys have only got a few minutes before the police come.
Here are some tips for the cables from a guy that has had a few.
Make sure they are solid.
I had a cheap one whose combination failed on me and in desperation, I tied to pull it out and was shocked to discover that under pressure, it just fell apart. I gave the next one a few big tugs to make sure that it worked. Also, make sure that the chain looks solid if it seems solid thieves will probably not even try.
Make sure that the chain is adequately bolted. After all the chain is only as good as what it is attached too and how strongly it is held to that object.
Make sure the chain is relatively long, a short chain will be hard to move around unless you have a stand.
Easy to use
Make sure the chain and lock does not interfere with the laptop operations, e.g. you do not want to lose a port or struggle with a switch. You also need to be able to put the laptop on a desk, some chains are designed to be used with a stand and so do not let the laptop sit on a desk.
Finally, for the lock, it has to be easy to use, I had one that could only be opened by picking up the computer and turning it upside down, it was a pain to unlock.
For the lock, I prefer combination, but I warn you that cheap combination locks are easy to break. If you are going to use a key, make sure you have spares. I doubt you can get a spare.