One common use case for theme authors might be to add a documentation link or an upsell link so users can find out about theme features.
The approach I like the best is one I first saw employed by the Theme Foundry with their theme “Make”. Their upsell link is prominent, but it doesn’t detract too much from the theme customization experience or take up too much space in the limited UI.
This tutorial takes a similar approach, but shows how to add a styled link to the “Active Theme” section.
To enqueue scripts specifically for the customizer, we’ll need to hook into the customize_controls_enqueue_scripts action.
We’re going to load one script “/js/upsell.js” and also output the link text in a way that will make it translatable (using the wp_localize_script) function.
You may notice that all the styles for the link are also added inline directly on the element. An alternate method, which might work better is to use the wp_add_inline_style inside the customize_controls_enqueue_scripts to add those styles inline at the head of the document.
I have some reservations about writing a tutorial like this because I’d hate to see the customizer ruined with custom markup that doesn’t match core styling, or even worse, huge upsell sections that make the customizer unusable.
As always, use good discretion, try to match core styling, and keep the end user in mind!
Update: Justin Tadlock also has a method for adding upsell links for themes. See on GitHub.