Ya, unfortunately I don’t see a super elegant solution.
A non-elegant solution would be to make a field of the “Number” type for each product you offer in your “Orders” table.
So you’d have a field (column) for “Spinach”, and a field (column) for “Lettuce”, etc.
Then in your form, the client could simply indicate the # of each product they want to order. The form would be significantly longer, requiring a header and entry box for each of your 25 products, and it wouldn’t be as aesthetically pleasing as there’d be no colors that you get with the “Multiple Select” field - but it would accomplish what you are trying to accomplish.
The problem is that with a “Multiple Select” field, there’s no way to tell another field which of the options you want to act on.
The inelegant solution I proposed would also allow you to price products differently, though, and use automatic calculation to total an order. Let me know if you think that’s something you’d like to try to do and you’d like more help with that.