That’s because Taxamo are a ‘deemed supplier’ like EBay or Etsy rather than an Intermediary. If the VAT collected isn’t right, only Taxamo are liable. Do they have a method of recouping under-collected VAT from their sellers?
Speaking of incorrect VAT amounts, I got a reply from the Swedish Tax agency. They confirmed the VAT must be collected on goods and shipping for IOSS consignments, as expected. Crossborderit are registered through Sweden, so I guess we’re getting different advice. Weird huh.