*** Solution ***

Hello.  I had the same issue, and I believe i have found what you are looking for.  Go to Sales on the menu and click on the product and services tab.  Then click the New button in the upper right area, and then click on bundle.  You can then make an "item" with multiple items or services.  So create a lawn mowing bundle with the mowing and treatment in one bundle that you can add to any customer/estimate.