Here is a link discussing the terms of the offer.

To celebrate Air China’s introduction of international online payment platform, PayPal, to its United States website, Air China is offering a special airfare discount for PayPal users. PayPal provides our customers with a more secure and reliable mode of payment.

From March 18 to April 18, 2011, passengers using PayPal to buy tickets on Air China’s United States website and spend up to USD 500 for each purchase, and begin their journey before July 31, 2011, can enjoy a discount of USD 50.00 or more…

…Passengers using PayPal to buy Air China tickets on the Air China United States website www.airchina.us and spend up to USD 500 per purchase will receive a refund of USD 50. Spend more than USD 500 per purchase and receive a 10% refund on each transaction with the highest per-transaction refund is USD 100.

Not a bad deal if you are heading to Asia in the near future. The offer equates to a 10% refund on your airfare. Note, Air China is also a member of the Star Alliance, so airline miles earned can be used in the partner reward programs.