ESN code

if its an ultimate game card like u said, listen up

log in to crossfire website (like on the top of this page)
hit deposit
from there click on prepaid cards
look for ultimate game cards
then select appropiate zp level ($20 means like 20k zp for $17 something)
buy or gift it (i buy)
it brings you to paybycash thing
input your email
then your esn (the pin u have to scatch off)
and make purchase
u will have to log in again to have it credited to your crossfire account
2 emails will be sent to ur email.
1 is the transaction
the other is paybycash pixels balance, to which when you have enough, u can use that to buy zp too