The test fee (and shipping cost if relevant) is paid to the laboratory directly, when the samples are returned to the lab. For most tests, you’ll be asked to complete your card details on a form which you’ll send back with your sample. Most labs also allow you to call in order to make payment.
For the FunctionalDx tests, a payment link will be generated by the lab, and this will be emailed to you. Payment directly to the lab is required before you go to get your blood drawn (for all FunctionalDx tests).