{"errors":[{"errorId":25101,"domain":"API_INVENTORY","subdomain":"Selling","category":"Request","message":"Invalid value for .","parameters":[{"name":"0","value":"err:216305|MailingBoxes"}]}]}
This means eBay doesn't accept the package type "MAILING_BOX" for the category this product is listed in. Every eBay category has its own list of allowed package types, and this one isn't on the list.
How to resolve:
Go to the product in WooCommerce.
Open the eBay tab.
Find the "Package Type" field.
Choose a different package type, for example:
PACKAGE_THICK_ENVELOPE
LARGE_ENVELOPE
PADDED_BAGS
Or leave the field empty if eBay doesn't require it for that category.
By choosing a package type that's allowed for the product's category, this error should be resolved.
Can't figure it out? You can always send a message to the discord community via the the bottom
