@Kim Duistermaat,
the prepaid system at the post office you mentioned is about a machine called La-ka-la,you recharge with the machine and it feedbacks you a receipt.first of all,u need to enroll a zhifubao account(chinese paypal…),then go to find a LAKALA near you to recharge your zhifubao account by paying cash.It’ll cost 2% of your total recharging amount for purchase.Not only post office,you could get a LAKALA in any Red-flag-chain-stores,Huhui-chain-stores and more.you can also buy a household LAKALA for your credit payment.
the LAKALA outlets in chengdoo
http://www.bankrate.com.cn/lakala/328____1