I now have a line in my "Billing History" that I did not have before. This is the line for "Promo code". Below there is an explanation:
"We'll use your promotional credit before we charge your method of payment for advertising".
But I did not get any promo codes. Maybe someone knows what that means?
If you're talking about AMS ads, about Sep. 12 AMS handed out a bunch of click credits to make up for a reporting glitch in the ad dashboard. They varied, based on a person's past ad campaign buys, but I think the minimum was $25. You needn't do anything, or have done anything, to get this.