Shipping cost being doubled under certain random product combinations.
One product, cost is correct. Multiple products of same type cost is correct. Add different product category item cost gets doubled, sometimes. Never tripled.
Once shipping is doubled adding items to get to next cost graduation doesn't work but adding enough items to get to free shipping works.
Any ideas, or can someone tell me what module to look at to try and troubleshoot in code
Are you using Warehouses? In your Shipping Settings, do you have 'Use warehouse location' checked?
As soon as I answered you I started checking and sure enough some products had no warehouse.
Dumb A__ attack. This is what happens when you copy a product that has incomplete data and don't check everything. Haven't tested yet but this makes sense why it was random.