I heard back from Nikki at EDS UK.

They are working on a better system for US residents to purchase their webinars.

In the meantime, she says hundreds of US residents have used this address at checkout without problem International House, 45-55 Commercial Street, London, E1 6BD.

If you prefer to contact her directly to work something out, here is her email address.

nikki@ehlers-danlos.org