Yes, I want people to be able to mix and match. For example, they might buy 5 plants (plugs) and 1 plant (9cm pot).
Plants are not the same price though. I think that's where I'm getting confused myself.
The cheapest you could get a plant for would be £0.92 - if you purchased 100+ plug plants.
Do I need to start with a base price for each plant, in the product table, and then apply and incremental price based on type of plant and quantity?
So, I'd basically need a "product type" table along the lines of...
typename Plug 0-14
typeincprice £x.xx amount
..and then a link table between that and the product table?