Build an Online Store

I currently have a desktop application (windows) I had previously put it up as it had a few bugs but i have now decided to move over to the internet option rather than put up with all the problems that a desktop app can give. Ok so I will provide a instal pack of current software so you can see how it works. Essentially it is a database that is used to send letters via my service, I install the software using the clients own excel databases. Each time the client wants to send out letters they use my software to generate and export (via email) a spreadsheet to me. Each time they want me to generate a letter they would send my the spreadsheet via my current software, this software also creates a history of client activity,, ie when letters letters where redeemed as well as this a client management module to check the activity add remove archive inactive clients etc This is facilitated by a barcode that my software creates from each mailout this barcode is scanned into software creating client activity the barcode included customer ID amount $ to be redeemed and expiry date of this voucher.