The MerchantFulfillmentOrderItemId must be unique for all items
in the same fulfillment order. This value can be used to
disambiguate multiple fulfillment items that have the same SKU.
If your system already has a unique order line-item identifier,
then that may be a good value to put in this field.

DisplayableComment contains a value that will appear in
recipient-facing materials such as the packing slip.