Nick here from Shopify.
I took a look at the Huel website and wondering from what you described and what's on their site if there are two different types of apps you could look into here, the subscription feature apps and the product options feature apps. Let's take a look at some of the most popular ones of them below:
Product Option Apps
After some investigation, I couldn't find an app that would truly incorporate the two of these into one app, so finding two high-quality apps each focused on their specialty could be your best bet going forward here.
Hope this helps!
All the best, Nick
Good question @andpow.
I don't see it being an issue. The apps wouldn't necessarily merge but both can be used and applied to your product pages and the customer would never know the difference. It would need to be an aspect of the product no matter what in your case, so adding both of them individually here doesn't seem like it would be an issue. So I could see them working together for the product overall, but not merging into one if that makes sense.