Receiving Invoice when I am in Free plan

I have been using convex for a while, I joined convex via Vercel's Next,js Global Hackathon. That free coupon expired and auto switched to a free plan. but today I received an invoice dated (7th of July and due of 8th of July). How is this even possible when it is being sent on 12th of July. Now I received a credit due email within a minute after invoice email. I don't see any anomalies in the Convex billing page tho. Can I get an explanation or get this issue resolved?