Please prepare CNY cash in advance, or exchange your foreign cash for CNY cash at the airport upon arrival in China.
Using Alipay is another way to pay in China.How to Use Alipay (Zhifubao) in China (chinadiscovery.com)
Website of Intercity bus:
PEK:BCIA
PKX:Beijing Daxing Airport Shuttles & Buses | Lines, Timetables & Tickets (daxing-pkx-airport.com)