It is a practice to write Shubha Labha along with the image of Ganesha. Shubh is ‘auspiciousness’ and Labh is ‘Profit.’ The two words are mainly written by traders, business men, shopkeepers etc on account books and in front of the shops. Some Hindu cultures consider Shubh and Labh as the sons of Ganesha.
In some Hindu traditions, Ganesh has two wives named Riddhi (material wealth) and Siddhi (intellectual and spiritual wealth). Ganesha has two sons named Shubh and Labh in Riddhi and Siddhi. (source:hindu-blog.com
So writing Shubh Labha is to gain auspiciousness and Profit and its also symbolic to Lord Ganesha's children name too.