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 tab. Copy the key under API Access.
In WooCommerce click WooCommerce > Settings > Integration tab > paste the key into the “Printful store API key” field > click Save.
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