Ultimately, I don't know who is going to use the platform. It's created in a way that it can be just as useful for a random person who wants to make a few invoices per month, someone who wants to make 10,000 payslips every 2 weeks, and someone who wants to hook it up to their product to pump in data and receive a link to a PDF via the API. The first person would not pay for a subscription, it's just not worth it at their usage level. The second has burst bulk usage which should be discounted. The third has some level of sustained usage over time.
Those 3 use cases are covered by the 7 days, 30 days and 2 years pricing structures. If you're a casual user, pay a bit more and don't worry about expiration. If you're a bulk user, you get a discount. Everyone else can treat it like a subscription and get the best of both worlds.
I looked to websites that do similar things for ideas, and the closest I could come up with was sites like ShutterStock which also have a credit system where the more you buy the cheaper it is. I believe their credits expire after a fixed period too, but you don't get a discount for how long which is a bit more specific to my product.
Thanks for the feedback! How would you prefer to see a service like this priced that wouldn't cause price anxiety? I'd love to hear different ideas since this can be changed.