I have found a couple of bugs that I have found in the Image store plugin.
First in _store/pricelist.php, $package_sizes is not getting cleared out in the foreach loop causing subsequent packages to contain the image sizes of previous packages. Added a line to clear it out to fix it.
Second in admin/pricing.php, the image size unit is in a column after the price displaying things like (4×6 $5.00 in) instead of (4×6 in $5.00). Appended the unit after the size to fix it.
Thanks for the great tool!