How to charge a product that could be a feature of a big company product?

Hi,

I am the developer myself and I am using a product of one big company. I noticed that I could create a new feature based on their API, that could help in my day to day work. I think that it could help other people like me since the situation is the same.

I am aware that the company could easily kill API or make the same feature in their product and that would kill my product, probably. I am OK with that as a possible scenario, since with this product. So, mostly the learning process, while I think that it should possible to earn some money since I think that the product could be valuable.

How I should charge for it? I do not want to make people fools and lie to them and charge an annual subscription since product could be shutdown easily by a big company.

Should I try only with monthly charges? If the product is shutdown then I do not own anything to the people.

Or maybe some fixed fee, but I guess no indie hackers like that.

What are your thoughts? How I should do it?