Both your header and your Page tabs are correctly aligned. WordPress.com is a multiuser blogging platform. Here all blogs wearing the same theme are using the same template. We cannot eit themes or templates and change functionality at all. Only Staff can access and edit and every edit they makes affects all blogs wearing the same theme.
The pages navigation links are positioned where the template is coded to display them. If you add more Pages to the top navigation the display will add them on the right hand side and the list of page tabs will move to the left to accommodate them.
With regard to the "Stationery Shop" do be aware that WordPress.com free hosted blogs cannot be used to drive traffic to third party sites. Also note that no blogger initiated advertising, retailing or reselling the work created or services provided by anyone other than yourself is allowed on free hosted wordpress.COM blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.COM.
If you have any further questions about advertising and/or affiliate links and/or ecommerce on a free blog from and being free hosted by wordpress.COM please contact Staff directly using this link. http://en.support.wordpress.com/contact/
If you do require an ecommerce site, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/