Pictoric,
I was able to duplicate the behavior you are experiencing.
The intent of the stored invoice items is to provide a consistent description and cost that can be easily selected by the user. However, you're right - it should work the way you are describing and I am adding this to the issue tracker right now. I did find a workaround in the meantime.
Once you've added an invoice item and selected a stored invoice item, modify the description and then click "add item." Another invoice item will appear above it, and the first description will be saved. Then you can delete the new invoice item.
Thanks for letting us know about this issue. It will be fixed in the next release.
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Rob
DotNetInvoice Support