Are you trying to buy the ticket on the China website or the USA website? The payment site should be the same but you may have better luck trying the USA website. You can pay by phone as well, not sure if they charge booking fees though.

My favourite China travel booking site is Travelzen (www.travelzen.com), a HK-based company which can also do flights for Singapore, Taiwan and China including international flights originating there. They don't always "find" the same inventory as Ctrip but they don't charge foreign card fees and their call centre speaks pretty good English (it is HK after all). I did a quick search for PEK-ALA and it definitely found the CZ flight through URC but for some reason it only showed full Y availability on the day I searched. You might want to give it a whirl nonetheless.

Despite being a CZ elite, I have actually only ever booked on the CZ website once before. Chinese airline websites are like ticking bombs waiting to explode, I avoid them at all costs. It's so much easier to do it on Ctrip or Travelzen.