Select add-ons at the checkout page when purchasing your challenge. They cannot be added retroactively. The total updates automatically so you can see the final price before confirming.
On the 1-Step Challenge, 2-Step Lite, and 2-Step Pro, add-on costs are paid alongside the challenge fee at checkout.
On Pay After You Pass, add-ons are still selected at checkout but the cost is paid alongside the activation fee after you pass, not upfront. Your $5 simulation fee remains the only upfront charge.