In Indian mythology, Shubh and Labh are sons of Lord Ganesha, the God of good and auspicious beginnings. Shubh means goodness and Labh means benefit.  It is believed that using the words 'Shubh Labh' pleases Lord Ganesha and Goddess Laxmi (the Goddess of wealth). Obviously, when they are happy, you stand a better chance of having good luck and benefits.  Writing 'Shubh' is like praying to God that whatever auspicious is there, it should come to our house.  Writing 'Labh' means that we should be benefited with whatever is auspicious and good for keeping the house going forward. It is like praying to God for benefits that will make the house run smoothly.  

You could place the Shubh Labh symbols in the Puja Ghar (prayer hall) or in the cash box, so as to maximize cash inflows.  Else, you could place these anywhere in the house, since these are considered to be very auspicious symbols in Indian mythology.