Is Excel Add-On An Infringement?

Dear Rich: I developed an app/program that can be opened within MS Excel to help business's organize and compile data. Even though it is an add-on to Excel, do I have any copyright or patent issues to worry about if I plan to sell this app? It's unlikely you would get a letter from Microsoft (or any other software developer) unless you either lifted proprietary code without permission, or you stepped on someone's software patent --  always a tough call in the crazy and perhaps soon-to-be-extinct world of software patents. You could run into problems if you are using a developer's kit or other licensed software and your add-on violates your license (also unlikely). Finally, you could run into a few hurdles if you advertised your add-on as being endorsed by or associated with Microsoft (or other software providers). Based on the high volume of Excel add-ons (or add-ins as they are more commonly known) the Dear Rich Staff feels that none of these issues will arise for you and you're probably going to have a problem-free release.