maximan on "[Plugin: WooCommerce - excelling eCommerce] Best approach to adding a single global optional extra to product"

I've read, and re-read, help notes about groups and attributes but could really do with some pointers. What is the best way to add a global option so a buyer can click a box on the individual product page to add a singe optional extra? TIA.

http://wordpress.org/extend/plugins/woocommerce/