Although Ric is correct in that it is more profitable to sub your work out to a smaller CPO...you did also ask about obtaining your limited license. This license will help you become more profitable by subbing out the lawn to the CPO while doing the shrubs yourself. The fertilizer certification which you are talking about requires you to take the Best Mgmt Practices, often referred to as BMP.
Both the BMP and your Limited license are able to be tested for and provided at your local county extension. They will of course be able to help you in determining if any other certification must be had in counties in which you work. The state will require anyone using fertilizer to have at least the BMP by 2014 I believe so I would advice getting it if you plan on spreading fertilizer of any type any where including the beds.
UF/IFAS Broward County Extension Office
3245 College Ave., Davie, FL 33314-7719
Phone: 954-357-5270 Fax:954-357-5271