Hi Glenn,
We've already answered your question via email, but to clarify for anyone else who might experience a similar problem, the "Does not exist" error happens when the product button is pointing to a product ID (or variant option combination) that does not exist. This can happen if a product's ID number is changed after the button is posted, for example.
The best way to fix such a problem is to get an updated button for that product from your E-junkie account and use that to replace the button that is producing the error.
We are available for support via email seven days a week during daytime business hours (based in Arizona, -700 UTC) and will answer all inquiries in less than 24 hours.