How do I create 
I can create all the html & css but not sure how to connect the liquid logic behind it.
I've tried with apps and it currently behaved like:
user adds 4 qty(4 metres of rope) and then selects how the end of the rope is treated (ie Heat shrink for $5). It adds the 4xQTY of rope to the cart with 4xqty of the $5 - Heat shrink.
Expected behaviour:
user adds 4 qty(4 metres of rope) with the dropdown addon of Heat shrink for $5.
It adds to the cart 4xqty Product and $5 once because the whole item is a single item.