Shopify app for upselling subscription-based items?

Yes, you can use Subscriptions by ReCharge or Bold Subscriptions to upsell subscriptions. For example, if a customer buys a monthly razor blades subscription, these apps will help you upsell shaving gel alongside their subscription. The opposite case would be when customers buy shaving gel and you want to upsell them razor blade subscriptions. That can also be done with the apps but will require a little more customization.