During checkout on the ‘Shipping and Billing’ step, after selecting the type of delivery and the delivery address, click on the section ‘Select an account'; there is a drop-down with options if you have multiple TAs loaded on your account:

The active ones will be selectable. You can pick the relevant account number and complete your purchase. 

If the account says 'inactive', please check that your account is in good standing. You could also log out and back in again after two hours and check again. If the account is still inactive, please submit a request here, or email onlineadmin@tarsus.co.za