Just pasting TnC from the website:
******
*1.Promotional price is valid for first year only
2.Can be applied only once.
3.Promotional price available only for the first domain, after that regular price, applies
4.Applicable only to new users.
5.Abusing this promotion by creating multiple accounts will result in domain and account suspension.
6. Domain will be locked for two years or until first renewal completion.
******
That last point is a huge problem for most domain investors. I'm not even sure if this is legal/allowed by ICAAN but there it is.
Also, the offer is available directly on the website as well (but we can use the AFF ID to help OP

)