Pay-as-you-go / Credit Rollover Model

I want a "top-up" or "pay-as-you-go" billing model where I can pay a set amount (e.g., $17) and use the credits whenever I want without them expiring at the end of the month. The current monthly reset system feels like a waste of money if I don't have time to use all credits in a single month. I want credits to roll over or just be a balance that I use until it's gone.