为什么堆栈中的值甚至不更新，它是一个参考

我有以下结构和类定义，我有一个问题:

struct customer{
    string fullname;
    double payment;
};
class Stack{
private:
    int top;
    customer stack[10];
    bool full;
    double sum;
public:
    Stack(){ 
        top=0; 
        full=false;
        double sum=0.0;
    }
    bool isFull(){
        return full;
    }
    void push(customer &c){
        if(!full)
            stack[top++]=c;
        else
            cout << "Stack full!" << endl;
    }
    void pop(){
        if(top>0){
            sum+=stack[--top].payment;
            cout << "Cash status: $" << sum << endl;
        }
        else
            cout << "Stack empty!" << endl;
    }
};

我在main中运行以下代码:

int main(){
    customer c1 = {"Herman", 2.0};
    customer c2 = {"Nisse", 3.0};
    Stack stack = Stack();
    stack.push(c1);
    stack.push(c2);
    c2.payment=10.0;
    cout << c2.payment << endl;
    stack.pop();
    stack.pop();
    return 0;
}

为什么总和不是12?我指定push构造函数为:void push(customer &c)。代码的输出是:

10
Cash status: $3
Cash status: $5

当我更新c2时，堆栈中的值应该被更新吗?付款给10?