You have a limit set up for how much you can charge to your room. You can alter that limit if you like. Once that limit is reached the card on file is automatically charged and your WDW balance resets to zero (and can accept more charging).
I was surprised once by this because my WDW balance at time of checkout was something like $190, after a week of normal shopping, dining, etc. I then looked at the list of charges and my balance had indeed reached its limit, my card on file had been charged, and the WDW balance reset to zero before reaching that $190 balance at time of checkout.
You can always go to the front desk before your purchases hit the limit and pay them off with whatever other payment medium you have.