(1) The journal entry to record the sale on March 12, 2024, using the gross method would be:

借：应收账款 - Cool Stuff Company 10,000 贷：主营业务收入 10,000 贷：销售折扣 - 2% Cash Discount 200

This entry assumes that the full invoice amount of $10,000 is recorded as revenue, with a corresponding discount of 2% ($10,000 x 2%) recognized as a contra-revenue account.

(2) If the payment is received on March 20, 2024, within the 10% discount period, the journal entry to record the receipt would be:

借：银行存款 9,800 贷：应收账款 - Cool Stuff Company 9,800

Since the payment is made within the discount period, the full 2% discount is taken.

(3) If the payment is received on April 5, 2024, after the discount period, the journal entry to record the receipt would be:

借：银行存款 10,000 贷：应收账款 - Cool Stuff Company 10,000

In this case, no discount is taken as it was not paid within the specified discount terms.