If you're a business or marketplace seller sending items abroad, compliant electronic customs data can only be provided through an approved shipping solution.
Electronic customs data is now mandatory when you send items/goods abroad (excluding personal correspondence).
Failure to comply is likely to result in delays, having items returned or even destroyed.
Depending on how you are posting your items internationally, this will dictate on how you provide electronic customs data.
For further information visit the Royal Mail website.