Every time you query our product API, your balance (the amount of available requests) is decreased by one request. Only successful (200 OK) requests are registered. If a request resulted in an error response (for example, 4xx), then it's not billed. Note that every successful request is billed, even if you make several requests for the same product.
When you register a new account, you start with a package of 250 requests for free. Your current balance is displayed in the account area.
If at some point your balance reaches 0, all subsequent requests will result in 403 error (see Product API — response codes and objects for more details). You can top up your balance by purchasing one of the packages here.