474370 C# building a key-generator and credit system

Hi, (watch) [login to view URL] I need someone to build a keygenerator and a credit. Must advise what method being used. Let me explain. (Just like a prepaid cellphone) You buy credit and put into your phone. Our software, whenever you use the software and click "PRINT" It will minus 1 credit from the total credit. To add credit you must contact administrator for new credit. Let me know if you can do it.