The base shipping cost is only applied to the cart if any of the individual product in the cart has a shipping cost specified. This way digital products do not get charges the base shipping cost.
Now what if you want to just charge the base shipping cost and not any additional charge for the individual items? Just specify a very low shipping cost value for your physical items (eg. 0.0001) this way the base shipping cost will get triggered but the shipping cost won’t change from individual item’s shipping.
Does it make sense?