Instructions on the email with the code:
If you live in one of these countries then pick "Premium Standard" as a shiping method to get free shipping:
United States, United Kingdom, Australia, Canada, New Zealand, The Russian Federation
If you don't live in any of the above then pick "Standard"
If the option doesn't exist then they either can't ship for free to your country OR the specific item that you picked can't be shipped for free (I THINK Yesstyle has other smaller sellers on their site - Just like Amazon does - and not everyone has the same shipping options)
In any case, you should contact Yesstyle support to be sure.