Using promo credits for pending bill

I’m using the free tier Atlas db
I got an invoice for Mongo Charts though, I hadn’t added billing info or promo credits earlier as I thought I’m just using the free tier.
I added the promo code I had, but I still got a mail again asking to add billing info or face account suspension. I wanted to know if it’d pick it up in some time or it’s not possible to use credits for this.