Brother using your credit card to buy steroids is just asking to have your name and address flagged when the place gets busted up. You have either the option of using cash at your gym or using bitcoin online. Even sending out western union to sources now has become a problem because you link your name to their receiver's name and just problems are bound to happen.
Being anonymous and not leaving a long paper trail is the only way to do it now in the year 2021.
Please let us know how your purchase went and if you got your stuff safely.