How can customers add one-time purchase products to their subscription?

If you want your customers with an active subscription to add one-time purchase products, simply go to: Seal Subscriptions > Settings > Customer portal settings > Product editing and select "Customers can add/remove products" and "Customers can add one-time products".

This way, customers will be able to add products to their subscriptions, which will only stay for 1 renewal. After the subscription renews, the product is automatically removed from the subscription.