I think the 90 days is what might have got you - I know for a fact what I have been told by the merchant guys and my accountant is that after 90 days is up I don't need to keep the register and EFTPOS receipts because after that a transaction cannot be disputed.
However, why would the bank alert you to the smaller transactions and then not question 2400 being spent on the same day - might be worth asking them the question.....