Hi,
I'll take a guess from recent experience with another seller using Cal'ed USPS rates and International?
Are you offering 1st Class International shipping? Since USPS doesn't provide Insurance for 1st Class International, eBay won't allow the Cal'ed rate to appear (at least not for Required Insurance). In eBay-think, there is no such thing as other Insurance carriers for USPS.
The work-around we discovered was to make the Insurance a Handling cost when required.
I don't know what offering Optional would do with International, but I can guess it would also have problems.
Danno