You can add your IOSS number in your Stallion account by going to the settings and selecting your store. From there, you can add your IOSS number.
There is no need to write the IOSS number on the package itself as this is confidential information which is stored electronically.
If you choose to upload CSV files from your store into your Stallion Express account, you will need to set up an IOSS Automation Rule to avoid your recipients paying twice for imported fees.
You can use this instructional to set up an IOSS Automation Rule on your Stallion Express account:
https://scribehow.com/shared/How_to_Setup_an_IOSS_Automation_Rule__4Ya5qao_QgOw0O-dOrjtug