Add the SIM to your shopping cart.
Enter code "STAFFSIMS" in promo codes on the right hand side.
Discounts the SIM to $0.
For multiple free SIMs you can add one SIM multiple times to the cart before checking out. Works fine with 2 SIMs. Probably don't abuse this.
Edit: Seems to be going in/out of stock to stop scalpers. If it's OOS, try again after an hour or something. Working fine for me as of noon NZ time (I'm AEST right now): https://cdn.discordapp.com/attachments/272384490297360384/61...