What is the version of eCart that you are working with? Did you use the checkout wizard? This sounds a bit like an old issue that was present in a previous release of eCart. The newest release should not have this problem. At the time of this post the release is 4.5.2. Creating a set of checkout pages with version should create pages that do not have this error.
If you cannot resolve this issue using the latest release please post back and include some info about the charge that you are implementing that is resulting in this error.