Treatment of the British Visa Premium from the United States

I asked for the treatment of a British tourist visa premium. I received a confirmation email saying that I had to provide biometric data. But before that, there is a series of steps, where I have to buy the courier service. I have selected my center as an Application Support Center in the United States, but it is stated that most email services are not available at the Application Support Center. Is premium processing not done at ASC? I've got my appointment for biometrics and I need to buy the mail service before that. Please, give your opinion.