This can occur in 3 key situations:

  • when the weight.unit value in multiple shipment_contents differs (e.g. one shipment_contents specifies kg and another specifies lb)
  • when the dimensions.unit value in multiple shipment_contents differs (e.g. one shipment_contents specifies cm and another specifies in)
  • when the currency value does not match across multiple shipment_contents (e.g. one shipment_contents specifies GBP and another specifies USD)