If you see an error message that reads ERROR: Invalid API key during checkout, the store key from Printful was entered incorrectly in WooCommerce’s settings.
To find the store key in Printful, click Stores → Edit (next to your store) → API. Copy the key under API Access.
In WooCommerce click WooCommerce → Settings → Integration tab, paste the key into the “Printful store API key” field and click Save.
