If you have 'free' API access then you are providing some benefit to the web service (whether you know what the benefit is or not), when you have a 'paid' API access you are a customer of that web service.
If you're paying for a service as part of your product, you need to understand how your costs may change over time, and generally you put into the contract language under what conditions and time frames the price can change.
This is one of the essential differences between building products and building demos. Just like an engineer might build a circuit that works to prototype something, they will go back and do all the edge case and tolerance analysis prior to putting it 'into production.' If you've never shipped a product before it can sometimes be a painful learning experience to get these sorts of lessons the hard way.