When selling using and IOSS registered marketplaces like Bandcamp or Discogs you need to set your Whiplash account to apply the correct IOSS number onto the relevant shipments.
If you only sell using a single market place, the IOSS number can be set on your Whiplash account.
Alternatively if you have multiple different retail partners you should utilise routing rules to ensure that the right shipments have the right IOSS number applied. Routing rules can take specific order attributes and allow you to set or remove an IOSS number.
If you sell using Ochre but also use Whiplash for fulfilment, the Ochre IOSS number will be added to the main account settings and used for orders as the default IOSS number. You then need to set up rules to ensure marketplace orders use a different IOSS number.
For Bandcamp it is optimal to create a rule that takes all orders with the "Originator" / "Provider" is Bandcamp and sets the Bandcamp IOSS.