1. The accountant confused the use cost of 10 day with the use cost of 1 year. The length of time must be converted to the same, so as to compare the costs of the two.

2. Give up the cash discount. If the cost rate is 2%/(1-2%) * 360/(30-10) = 36.7% >12%, you should enjoy a cash discount.

3. If the company is forced to defer payment and pay on 15, the cost rate of giving up the cash discount is 2%/(1-2%) * 360/(15-10) = 48.98% > 36.7%.

Therefore, payment should be made 30 days after purchase, so that the annual interest cost can be reduced to 36.7%.