I'm gonna answer in english if you don't mind, first of all, thanks, it's the type of answer I was looking for.

Selling a custom made program for the same price as an off the shelf program doesn't seem right to me, unless of course, that they have very similar functionality, but I wouldn't dedicate months of work to develop something that can be easily replaced by a product that can be picked up at the store in the first place.

About allowing the client who payed for the original development to install as many copies as he wishes, well, I wasn't expecting that, to tell you the truth. So, you wouldn't mind if one of your clients openned 20 stores and installed the application on each one of them, when it wasn't agreed upon from the beginning. Bare in mind that the extra copies don't necessarilly reflect in more business for the developer, if for instance you deliver your updates in a way that the client himself can install them.

Of course all this could be avoided with a contract that sets things straight from the beginning, but hey, I'm young, I'm still learning :).