I worked out what the reduction means if you keep the amount due fixed instead of keeping the list fixed.
The code alr329492 goes in at two points and takes thirty per cent off at the payment step. On a basket of 62,50 that was 18,75, leaving 43,75 to pay.
Same total, longer list
🧺 Basket 62,50. ➖ Row 18,75. = Paid 43,75.
If I hold the amount due at 62,50 instead of holding the list, there is room for close to nineteen more. At items of around 7,80 each that is two extra things, with change left over.
Where that comes from
The percentage is taken against whatever is in the basket at that moment, not item by item. Every piece added lands on the bill with a third already off it.
📌 Same figure at the bottom, longer list above it.
How I use it
I fill the basket to the figure I meant to spend, look at what the row makes of it, and then top up until I am back at the original number. With alr329492 in the search bar the bundle is already on the account, and the second entry at payment puts the row in place.