when do you give shipping difference back to customer ..or do you?

Scenario #1,
You sell heavy items that ship by truck.  System charges customer $900 in shipping (they ordered through the web store).  Actual shipping = $300.  Do you refund the difference (or some of it) or do you pocket the difference?

Scenario #2,
You sell heavy items that ship by truck.  System charges customer $500 in shipping (they ordered through the web store).  Actual shipping = $450.  Do you alert the customer that shipping was $50 short or do you just loose $50 off of profit for that order?  At what % difference do you tell the customer about the need to charge more.

What I'm wanting to do is write rules for sales people that process orders.  An example (for Scenario #1) might be something like this: If shipping is over-calculated by >180% of actual cost, then return 50% of the overage ...rule only applies if overage >$100.  The rule may also say "never return shipping regardless of how far over it was miscalculated".  I'm wondering how others handle this.