Failed to publish offer: The eBay listing associated with the inventory item, or the unpublished off

[bsfs]

I keep getting this error when trying to publish to eBay from Shopify

"Failed to publish offer: The eBay listing associated with the inventory item, or the unpublished offer has invalid data in the associated Fulfillment policy. At least one valid postage service must be specified."

It has been working fine for years and I haven't changed any policies or anything so I've no idea what to do?

help appreciated.