"subscriptions are set up by customers during checkout."
So when you think you have ordered something, and are focussing on paying, you miss that if you don't check "one off", they'll just keep coming. Then you learn another is on its way, and lo, it is too late to cancel because it's on its way. How many people have never used you again after that experience?

The reply confirms that the default setting is continuous deliveries, and that if you don't spot, and click on, "one off" at the end, just before you pay, you get continuous deliveries. No "number of emails" were received. Only a notice that something was being delivered, and a statement that it was too late to cancel. Do you care about your customers enough to have "one off" as the default option???