It's easy to set up an email link using the Yahoo! Store Editor. Select "New Link" from the Editing Toolbar. Be sure you've included a complete and valid "mailto:" address in the "URL" property for an email link.
For example, to create a link to the email address youremailaddress@yahoo.com, be sure to enter the complete email URL:
mailto:youremailaddress@yahoo.com
rather than a partial URL such as:
youremailaddress@yahoo.com
If you are familiar with HTML, you can also create the link manually. You can enter HTML in the large text-entry fields within your store, such as the Message, Caption, and Final-text fields.