"...or that all plugs for any plant are the same price, and all 9cm pots of any plant are the same price, but different to the plug price."
That's almost correct, but the price changes depending on the quantity purchased.
All plug plants are £1.15 each unless you buy 15, in which case they drop to £1.00 each, or if you buy 30 then it drops again to £0.98...and so on (see discount scales in previous post).
The discount, therefore, isn't a percentage it is a flat rate that is dependent on the quantity chosen of any particular product type.
The flat rate discount, however, isn't constant across all types of product (plug, 9cm pot, 1 litre pot).
For example, the plug plants are £1.15 each, unless you buy 15, at which time we want the price to drop to £1.00 - that's a drop of approximately 13%.
However, the 9cm pot plants are £1.75 each, unless you buy 15, at which time we want that price to drop to £1.60, which is nearer to a 14% drop.
So, basically, we want to apply a flat rate discount that is dependent on the quantity chosen of any particular product type.
I'm going to try it with some database integration and see if I can get it to work with a flat rate discount based on the product type. As you mentioned earlier, I think this will mean that I'll have to add a new column to eCart to handle the product type variable, in order to calculate the flat rate discount based on quantity ordered.
Appreciate the help, thanks.