Various aspects of the shopping UI are strange, and the rules on what quantities of products can and can’t be ordered together in an order are truly bizarre. Some earlier threads about this:
Plenty of other sites get this right with the same, simple formula: the site sells several products. In one order you can buy any combination of products in any quantity, with no minimums. Buying one thing doesn’t affect whether you can buy another. No subscriptions and resulting complications. The rules are the same for all orders. But no, not Huel. It’s ironic that a product which drastically simplifies one’s life otherwise (no cooking! no grocery shopping! no washing dishes!) makes its online shop needlessly complicated.