Emails don't cost "next to nothing." They might not cost much, but they do have a cost.
I've had to deal with enough email marketing, I still work closely with the email guys. Heck, even my job, you could technically say that creating a new targeted content page "doesn't cost much" (until you actually look into it). But then you actually add up the cost.
What other costs would be associated here? Well, first you have to create a new product ID/SKU for the subscription and add it to the database. Then you need to program behavior to auto-deliver products to people who've purchased said ID. You also need to make sure that anyone who's purchased it doesn't get targeted advertising recommending they purchase something they've already effectively purchased, so all those other product IDs have to be told to recognize that particular ID as a prior purchase of that product. All of that takes times and resources. It adds up.
My job is ecommerce. I'm involved in email marketing, web marketing, I work with the guys who run our website, I know the costs associated with this kind of stuff. It's not as negligible as you think.