Here is the update from eBay for customers receiving the validation error = Postage service Standard International Flat Rate Postage(51507) isn't available.; zeroed out
As we shift to be more aligned with the international postage services supported on eBay, we recently renamed certain services to better support cross border trade from all countries. Due to this update, some services were inadvertently impacted.
The services impacted were:
•Standard International Flat Rate Postage  - this service has now been re-instated
•Economy Shipping from China/Hong Kong/Taiwan to worldwide (11 to 35 business days)  now renamed 'International Economy : untracked (11 to 35 business days)'
•Standard Shipping from China/Hong Kong/Taiwan to worldwide  now renamed 'International Standard : tracked-signature (7 to 19 business days)'
•Expedited Shipping from China/Hong Kong/Taiwan to worldwide (2 to 7 business days)  now renamed 'International Express : tracked-no signature (3 to 7 business days)'
•RU Tracked Packet from China  now renamed 'International Standard : tracked-no signature (7 to 15 business days)'
We apologise for any inconvenience caused.
Fortunately, the issue has been resolved and you can now relist any listings that were impacted. Please contact eBay customer service if you have any questions.