You are most likely a small business taxpayer so you can choose not to keep an inventory and simply use a method of accounting for inventory that clearly reflects income.
See https://www.irs.gov/instructions/i1040sc#en_US_2021_publink24329wd0e2328
In your case, you were obligated to pay the vendor when you received the merchandise in 2021. A method that would clearly reflect income would be deducting the cost of the inventory you actually sold in 2021.
