I set the price $19.99, tax 0%, and Free Shipping Options, then someone ordered the products, he got the product at $19.99. the problem is in my zen cart admin, the details of the order is:

Price (ex) Price (inc) Total (ex) Total (inc)
$24.80 $24.80 $24.80 $24.80

Sub-Total: $24.80
Free Shipping Options (Free Shipping): $0.00
Total: $24.80

why is this happen ? the system email sent to the customer also say the price is $24.8, but the real price is $19.99.

Please help !