Thank you for the provided URL.
The "Sign in" link is displayed on this page, because there is no Single Sign On feature on this page.
As far as I understand, you have created a separate
http://thesewingspace.com/zip-sale/ page and added Ecwid integration code manually to it, while the store itself (
http://thesewingspace.com/store) has been created with the help of Ecwid
plugin, built into Wordpress. When you have enabled Single Sign On feature, it has been applied to Ecwid plugin only, not for the pages you have added Ecwid code manually to.
I presume you would like to place the link to
http://thesewingspace.com/zip-sale/ page somewhere in the site menu, correct? If so, you don't need a separate page for this category. You just need to add a link to this category, located inside your store, but not as a separate page. In this case the category will belong to Ecwid plugin and will inherit all the settings you have for this plugin, including Single Sign On feature.