This problem may result after a modification of the website carrier :
at each change in the carrier settings , Prestashop assigns a new ID to the database, while maintaining the settings of the previous carrier, this also concerns simple changes to ranges, prices or names of the carrier.
After each website modification it is always necessar to go to Fastbay “Shipping” tab and perform a configuration save / update , so that Fastbay database can be refreshed with the information of the new carrier.

If this does not solve the problem please check :

• That the carrier is NOT disabled in Prestashop or is not managed by a third party module that could bypass the “mapping by zones”.
• That the weight/price range used in Fastbay for carrier mapping is compatible with the country zone of your eBay customers and the products you are selling.