[Solved] Parse stringified JSON

do it in two steps.

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    type AutoGenerated struct {
        Messages         string `json:"messages"`
        AppCart          string `json:"app_cart"`
        Addcartrows      string `json:"addcartrows"`
        MinicartContent  string `json:"minicart_content"`
        CartQty          string `json:"cart_qty"`
        AddedProductJSON string `json:"added_product_json"`
    }
    type addedProduct struct {
        ID   string `json:"id"`
        Size string `json:"size"`
    }
    type productRow struct {
        ProductID string `json:"productId"`
        Quantity  int    `json:"quantity"`
        UnitPrice string `json:"unitPrice"`
        Currency  string `json:"currency"`
        SKU       string `json:"sku"`
    }
    var j AutoGenerated

    body := []byte(`{
 "messages":"<ul class=\"messages\"><li class=\"success-msg\"><ul><li><span>Item succcessfully added<\/span><\/li><\/ul><\/li><\/ul>",
 "app_cart":"[]",
 "addcartrows":"[{\"productId\":\"1675688\",\"quantity\":1,\"unitPrice\":\"290.00\",\"currency\":\"EUR\",\"sku\":\"P00525485-3\"}]",
 "minicart_content":"<ul class=\"checkout-types minicart\">\n<li>",
 "cart_qty":"1",
 "added_product_json":"{\"id\":\"1675695\",\"size\":\"40\"}"
}`)

    if err := json.Unmarshal(body, &j); err != nil {
        panic(err)
    }

    var k []productRow
    if err := json.Unmarshal([]byte(j.Addcartrows), &k); err != nil {
        panic(err)
    }

    var u addedProduct
    if err := json.Unmarshal([]byte(j.AddedProductJSON), &u); err != nil {
        panic(err)
    }
    fmt.Printf("%#v\n\n", j)
    fmt.Printf("%#v\n\n", k)
    fmt.Printf("%#v\n\n", u)
}