I'm adding modifier group by modifying inventory Modifiers in the sandbox dashboard.
When I try to "Make a copy" of a modifier group and edit the title/prices, there is no save option. Exiting this edit screen will not save the copied modifier group. However, sandbox dashboard does allow me to "Add New Modifier Group" and save it. You'll need to make all the correct edits and modifier while in the edit screen the first time. I cannot add or remove modifiers afterward.
Is this intentionally left incomplete?
EDIT: After "Add new Modifier Group" and adding each modifier. The Save button got push down the page and disappear. So you can't really do anything in the Sandbox Inventory Modifier groups.
I guess a better option is to create inventory and modifiers in the regular dashboard, export and import to sandbox.
