If you didn't use accrual accounting method and track a cost of good sold account in your accounting records, then you shouldn't use COGS on your tax return. As long as your total sales were less than $1,000,000 for the past 3 years (or $10,000,000 in a second exception), then you are not required to use 'inventory accounts."
Instead, you can just deduct all the costs of purchasing inventory, under the expense category supplies. To read this info at the IRS see the "inventories" and "qualifying taxpayer" section at the following IRS resource: https://www.irs.gov/publications/p334#en_US_2017_publink1000313278
