Access Promo Code logic via SuiteCommerce Advanced Services

I'm trying to figure out some best practices on a somewhat novel use of promo code logic in our SuiteCommerce Advanced install.

We have a customer who needs to be able to see what a promo code would do to product pricing BEFORE they add it to their shopping cart.  Basically our codes modify pricing / add-ons SKUs at different quantity levels, so we want to present all those quantities to the user then let them add the right one to the cart.

This kind of backwards flow (promo code entry before product is in the cart) is giving me some headaches.  My inclination was to create a SuiteScript to read out the promo code logic and basically mimic the same logic we wrote into the backend.  However, when I try and access promo code records from SuiteScript (i.e. /ShopFlow/services), I get the error "Permission Violation: You need the 'Lists -> Items' permission to access this page. Please contact your account administrator.".  Since these services don't have normal "script" administration like Suitelets or RESTlets, I'm not sure how to set those permissions.