Need help with a permission error, please. Trying to do a search from an extension on a custom item field when user is not logged in. What role does SCA execute from? Is this possible?

Do you know item ids beforehand? If so, instead of doing a search, use the commerce api to get item values. If you don't, you'll need to elevate permissions on the service. The shopper role is not a role you can add permissions to. The permission the elevated role needs to have is "item" -> View.

Ok, thanks @PabloZ. Related question, one of the fields is a multi-select and I get back item names, not ids. Is there a way to do an item lookup using the name field with the commerce api?

No, only ids.

This is a bummer. Thanks for your help.