I have used phpCoin and it does have some quite good functionality considering that it is free software. However, as with most things, I have heard that the paid software has even more features and is easy to integrate. Also the pricing doesn't seem that bad so if you've got some money coming in I would think it is definitely worth the investment. After all a strong payment system gives clients the reassurance they need to trust entering their details on your web site hosting and therefore shopping with you. If they don't have this confidence and they don't order it means that you lose sales which will ultimately lose you more money in the long run.
Hope that helps
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