Sale Notification emails can only be disabled for all orders in general; we have no way to disable them for specific products or checkout methods. However, you might consider setting up an email filter for messages From:notifications@e-junkie.com which contain the phrase "Free Checkout" -- e.g., if you use Gmail for your domain, you could search your mail for this:
"Free Checkout" from:notifications@e-junkie.com
...then click the More button to select Create Filter, then click "Create filter with this search", then you might tick the boxes for "Skip the inbox (Archive it)" and "Mark as read" (and optionally tick the box for "Also apply filter to matching conversations" if you want the filter applied to past emails), then click Create Filter.
