I'm not sure that "conforms to your financial accounting treatment of inventories" means deducting when purchasing. Here's another link I could find on this. I was thinking more of any inventory method they choose and on a cash basis (so if they paid for items that they didn't receive yet, it would be included in inventory)

https://www.justanswer.com/tax/nd448-taxes-read-tax-cuts-jobs-act-2018.html

This is the first year, so I want to make sure I make the right selection.