OP, I'm not sure what you think happens when you go through customs - looks like you've cruised out of FL before so not sure why you are referencing customs and your KTTW card. They are not
DCL and they do not have access to DCL systems so there is no swiping of a KTTW card to see how much you spent on board. Two things would be wrong if that's how they did it: First, not everything purchased on board would accumulate to your $800pp allowance - services like spa do not, port adventures do not, drinks consumed on board do not, etc. How would they know if that $40 charge was for a bottle of alcohol you are bringing back or for a case of water you consumed on board? Second, it would not include any purchases you make off the ship that you will be bringing back. It gets further complicated in Custom's definition of household because you may have had separate on board accounts but your $800 customs allowance may roll up. If there are 4 people in your "household" you are allowed a combined $3,200. Customs wouldn't be able to or want to take the time to mix and match all the charges even if they had access to DCL's system.
If you really will be going over your customs allowance, there used to be a meeting place on board (customs official is on board long before they start the debarkation process). I have never gone over our allowance so I don't keep up with it so don't know it that's still how they do it.