I am getting a pretty low newsletter subscription rate on my Magento 2.1 store,
I'm looking for an extension that will add an already checked newsletter subscription box when someone buys a product. e.g. without the person needing to create an account.
There is a free plugin available for it: See if this helps: https://marketplace.magento.com/mageside-module-subscribe-at-checkout.html
Or you could just Hire Magento Developers to get everything done at once. A much easier path I should have taken a long time ago.
One more suitable option is the Smart One Step Checkout extension for Magento 2 by Aheadworks. If necessary you can configure the module just the way you need, when the newsletter subscription checkbox is checked by default.
Still, customers are able to disable the subscription anytime during the checkout process.
Product page - https://goo.gl/jgxrhW
Configuration section in demo store - https://goo.gl/s54Vtx