Allow customers to add items to a subscription?

I operate a meal delivery business. Most of my customers are on mobile devices, and I'd like to save them the trouble of having to manually enter an order every week.

The problem is that if a customer wants to order an add-on for a given week, there doesn't seem to be a way to combine it with their existing meal plan subscription. I'm not very familiar with the plugins that are available though, so perhaps there is a solution?

For example, Jim orders a subscription Meal Plan A, which will be delivered to him weekly. On the second week he decides to order a few protein bars. How can I combine this order with the weekly subscription fulfillment so that he is not charged for delivery twice?