Cash accounting method and cost of goods sold

I have a side business with about 20K of income a year.  Mainly a service business.  I do sell parts to people, but I keep no inventory on hand.  When someone needs something I will order it and invoice them.  I use the cash accounting method. 

How do I handle cost of goods for something when for example I sell something at the end of the year.  I order the part, and pay for it, but I do not receive the check until the next year.  

Using the cash method, I wouldn't report the income until the 2020 tax year, but what about the cost of goods since I paid for the part in 2019.