Pay-as-you-go / One-time credit purchase system with non-expiring credits

I want a one-time credit purchase (pay-as-you-go) system. My usage isn’t consistent enough to justify a monthly subscription. Some months I may only use around 200 credits, while other times I might need 5,000+ credits. Because of this, even a plan that includes 3,000 credits per month doesn’t make sense for me. If I only use 200 credits in a given month, the remaining credits go to waste. A non-expiring, one-time credit system would be ideal, allowing me to pay only for what I actually use without worrying about unused credits expiring.