Entering Exact Package Size for an individual item--Shipping cost underage

[cycletrader1]

I have not found a way to enter exact package sizes when I add a new item.  You can only select a general box size in shipping.  This often means that even when the weight is correct, the shipping price offered to a customer, can be way off what the actual shipping cost to me is. especially on large box sizes.    Just bumping up the weight of the item is not a viable solution.  So my only option is to absorb the price difference, or get back to the customer and ask for additional money to ship the item(Not a good practice)

I cannot go back and select a box size for each of the thousands of items I already have listed.

Ebay offers you the opportunity to list the actual box size when you enter a product. I don't understand why shopify cannot do the same!!!  Someone please help.  Thanks