Reply To: optional product during checkout

That’s not possible at the moment. From Subscription documentation …

Variable Products and Subscriptions
Please note that subscription plans do not support variable product options due to some subscription API limitation at the moment

You can create multiple plans with the different price points and then offer them on a landing page. You can create as many product entries as you want to in our plugin and then embed them on your landing page.

For example: lets say that you want to offer a $30 per month option and then a $300 annual price option.

You can create one subscription plan for the $30 per month option and create a product for it. Then create another subscription plan for the $300 per year option and create another product for that. Embed both of these subscription products on your landing page. Your customers can then pay using the option they want.