eBay Post-Order APIVersion 2.7.4
 

Applicable values for /post-order/v2/return/{returnId}/add_shipping_label POSTRequest.labelAction

AU_LABEL
This value is used if an Australian seller has created an eBay shipping label for the return shipment. Australian sellers are billed for eBay shipping labels on their monthly invoice.
EBAY_LABEL
This value is used if the seller has created an eBay shipping label for the return shipment.
MARK_AS_SENT
This value is used if the seller is using the POST /post-order/v2/return/{returnId}/add_shipping_label method to mark an eBay-generated label as being sent to the buyer.
NO_LABEL
This value is used if the seller is not providing an eBay shipping label to the buyer. If this value is used, the seller must also include the noLabelReason field and provide the reason why no label is being provided.

It could be the seller is providing their own shipping label to the buyer (and not an eBay shipping label). In the case where the seller provides their own label, they must populate the labelId field with the shipment tracking number of the shipping label they're supplying to the buyer.
PURCHASE_LABEL
This value is used if the seller has purchased their own (non-eBay) shipping label for the return shipment.
UNKNOWN
Indicates the label action is not known.
UPLOAD_LABEL
This value is used if the seller is providing their own shipping label to the buyer (and not an eBay shipping label).