As Zoe said, this is a forum for technical questions, and for authoritative answers on legal matters, you should talk to a Microsoft licensing specialist.
However, this is one of the few license questions I dare to answer with confidence. This is absolutely the scenario where Developer Edition is a fair game.
There is one thing to watch out for, though: With Developer Edition, you have all features that are in Enterprise Edition. This can bite you if your customers will run on Express and Standard Edition. These days, the difference is mainly related to performance, but nevertheless you can run into to some puzzling surprises. So from this perspective, it may be better to develop on Express Edition. On very least, you should test on Express to make sure that your software runs on this problem.