When this happens, a very common problem is related to the fact that module shipping settings are not being saved after changing carrier settings in Prestashop. With each modification of a carrier setting, Prestashop assigns a new ID to the database, while still maintaining the settings of the previous carrier, this also concerns simple changes to the carrierr’s ranges, prices or names.
Although Fastbay handles these changes automatically, particular configurations or situations may require a manual SAVE on shipping settings afterwards.
Go to the module and save the configuration to update the Fastbay database with the information of the new carrier. After that it will be necessary to re-synchronize the products to Ebay to apply the new settings.