Inkthreadbale , prime print-on-demand service provider in the UK fully supports IOSS scheme.
If you use Inkthredable to make your merchandise, please , follow this link to setup your IOSS number with this print-on-demand service. Please, follow this link to setup your IOSS number in the Inkthreadable dashboard : https://help.inkthreadable.co.uk/en/articles/6221605-how-to-submit-your-ioss-number-for-eu-exports