Coupon code for a purchasable item.
coupon: undefined | string;
The shipping tier (e.g. Ground, Air, Next-day) selected for delivery of the purchased item
shipping_tier: undefined | string;
A context-specific numeric value which is accumulated automatically for each event type. Values
can include revenue, distance, time and points. When a value is set, the accompanying
parameter should also be defined.
value: undefined | number;