后端签名拼接：

appId={小程序appId，在此屏蔽}&nonceStr=1963315987&package=prepay_id=wx20200105205129596033&signType=MD5&timeStamp=1578228692&key=f1945e80663c896141429e4552af69f6

key是通过后端调用getsignkey API获得

prepay_id是通过后端调用unifiedorder 获得

小程序端调用支付API，却一直显示“JSAPI缺少参数total_fee“

wx.requestPayment(
{
    'timeStamp': '1578228692',
    'nonceStr': '1963315987',
    'package': 'prepay_id=wx20200105205129596033',
    'signType': 'MD5',
    'paySign': '4A523CC4A6EA958D166F2120E66A4FF7',
    'success': function (res) { 
      console.log("payment success ", res)
    },
    'fail': function (res) { 
      console.log("payment failed ", res)
    },
    'complete': function (res) {
      console.log("payment complete ", res)
     }
})