My name is Lilith, I'm on the Social Care team at Shopify.
It sounds like a pretty great deal for your customers!
I've got a few recommendations on how to build this promotion on your store.
There are 2 apps that can create customized bundles based on what the customer puts together. They will definitely do what you're looking for.
I also want to suggest Bold's apps as well below. Depending on your product or service type you can benefit from a "build a box" type of promotion. If you use the app below you can have your customers purchase a one time or a recurring order of products in a bundle with a set price. If you're ready to scale up your revenue, you can use this strategy to help!
One other app that I can recommend below will help you create premade bundles with set prices, but your customers might not be able to customize their own bundles. This could still come in handy if you don't have too many products in your store but you can create pre-set bundles.
As an alternative option, we have some incredible partners in the Experts Marketplace that can also build some code customizations for you. They're super talented and know the platform well, so if you want to just create your own unique rules for bundles you can have one of them do that for you.
Let me know how you do with the information above! I'm happy to answer any questions you think of as you work on your business.