请教如何从Json格式字符串取出某值，如图，取出以下字符串中的80
字段
"{
""fields"": [
{""necessary"":1,""options"":[""服务费"",""代收货款""],""name"":""feeType"",""label"":""服务费"",""type"":""select"",""value"":0,""desc"":""收款类型：""
},
{""necessary"":1,""name"":""payAmount"",""type"":""text"",""payCode"":""1245997416365297664"",""value"":""80"",""desc"":""收款金额：""
},
{""necessary"":1,""defaultValue"":0,""options"":[""微信支付"",""支付宝支付""],""name"":""payChannel"",""maxVersion"":""1.1.10"",""terminal"":2,""type"":""select"",""desc"":""收款方式：""
}
]
}"
