Is it permissible to add a new service as part of the developments of an existing service on the website, which the website refuses to list as a standalone service, knowing that the website will take its commission and that this satisfies the buyer, but it violates the website's terms and conditions?

The ruling depends on the terms of the contract between you and the intermediary website. If the website allows requesting a service other than the one offered by adding it to the developments, then there is no harm. Otherwise, it is not permissible, because the Prophet, peace and blessings be upon him, said: "Muslims are bound by their stipulations." If the questioner is in doubt, he should refer to the website administration and ask them.