Developers use the API to check stock, place an order, and read the SMS without clicking through the website. Create the key in the dashboard. The path list and examples are on the API docs. The API overview explains the product limits in plain language.
The API charges the same customer-facing price as the website for that country and service. There is no unpublished “cheap API” rate. If the row is out of stock, the call cannot invent a number. Wallet balance is the source of funds, and a short-term timeout with no SMS returns the charge the same way a manual order does.
Do not put the key in a public page. Do not use the API against accounts you do not control. Those rules are in the acceptable use page. Keyword-tool demand for “SMS verification API” is an estimate, not an uptime figure.