I don't do catering, but I am self employed in a service industry.
Your invoice can be pretty much whatever you want it to be. Mine is very simple. As long as my invoice is the same as what the customer expected it to be, my customers are fine. If you are thinking that you need to itemize the chit out of your invoice, to justify your price, you are more than likely over-thinking it. The important thing is to tell customers, up-front, what the gig will cost, and tell them, up-front, that if they ask for more than what was agreed upon, it will cost x-amount more.
From my experience being self employed most of my adult life, you only need to explain your invoice if it is more than the customer expected.
If you give customer an estimate, with clear boundaries, then you don't need to justify anything on your invoice.
Itemizing a gig is more important when you do your income tax return, than when you write an invoice, IMO.