Why are IOSS Numbers not appearing on my Labels?

IOSS numbers, whether it's a marketplace's IOSS number or your own, do not get placed on postage labels or CN22s/CN23 custom documentation.

IOSS numbers are submitted to couriers electronically.

For Royal Mail labels, when an order is subject to IOSS, a symbol is placed on the address labels to indicate that an order is an IOSS order and that the IOSS number has been passed electronically.

So provided that your shipping label has this logo on, the IOSS number has been passed correctly.