Feel free to mark it up but be prepared to answer your customer. Suppose they see the fertilizer you use and then go to the store and see the same product and price. The next question they may ask themselves is "what is my lawn guy charging for me for this fertilizer?" Now they compare your invoice with the price at the box store and see you are "over-charging" them. How do you think they will feel?
Solution: charge a little more on labor or find the product cheaper in bulk or make less profit or etc. You get the picture. You can do whatever you want but be prepared to answer for your actions. A good answer for charging above retail is that you have to spend your time and money on high priced gasoline to go to the store and purchase the product and for that you charge a justifiable premium but if they want to spend their time and money running around town then by all means have them go buy the fertilizer and you still charge accordingly for the "installation". The most important factor to remember is to give your customer the choice if they question you. Most people want value over low prices and as long as they know they are getting a good value for their hard earned money they will usually will not have a problem paying your price.