Well from what I know so far, the client would probably separate the delivery, because a large table or bed may take time to delivery, yet a small candle could be sent same day from the local post office next door to clients shop. The customer doesn't need to know this at checkout, once the client gets the order then they can determine best delivery method (which of the 3 delivery companies).
I think first I should check the basket if it's only one item, as that calculation would be straight forward and relatively easy to do. Does that make sense in terms of logic?
But then my problem is when there are multiple items in basket. Would it be best to categorise the products as small, medium and large (stored as a value of 1,2 or 3) in the product record. Then in my calculations group the products into their corresponding size tiers and then calculate their weight and then height, width and depth) and determine the cost value that way?
I'm just not convienced that I am doing the right type of calculations. Because if I categorise the products as either small, medium or large (which relates to the 3 different delivery companies, then if I have several items as small then their total weight and size may then full into the medium delivery category. I'm so confused. Are you or anyone can provide proven methods?