Hello Kay and David,
Thank you so much for reaching out to ShippingEasy.
If you're setting up your WooCommerce store integration now, you will no longer need to use the plug in or store API. We have updated the WooCommerce integration so that we can better support the integration with WooCommerce and offer our additional features for you!
I hope this helps resolve the issue so that you can be fully integrated with ShippingEasy!