compiler error: is private within this context

I'm writing a class and when I compile, I get one error message that says, "is private within this context" and another that says, "invalid use of non-static data member". But if I comment out everything before the addShipment function in my cpp file, it compiles just fine. Can someone please explain to me what is different to sometimes cause an error and sometimes not, and if the two different types of errors are related.

Product.h:

#ifndef PRODUCT_H
#define PRODUCT_H

#include <iostream>

class Product {
    public:
        Product(int productID, std::string productName);
        std::string getDescription();
        void setDescription(std::string description);
        std::string getName();
        int getID();
        int getNumberSold();
        double getTotalPaid();
        int getInventoryCount();
        void addShipment(int shipmentQuantity, double shipmentCost);
        void reduceInventory(int purchaseQuantity);
        double getPrice();
    private:
        int productID;
        std::string name;
        std::string description;
        double totalPaid;
        int inventoryCount;
        int numSold;
};

#endif

Product.cpp:

#include <iostream>
#include "Product.h"
using namespace std;

Product::Product(int productID, string productName) {
    Product::productID = productID;
    Product::name = productName;
}

string Product::getDescription() {
    return Product::description;
}

void Product::setDescription(string description) {
    Product::description = description;
}

string Product::getName() {
    return Product::name;
}

int Product::getID() {
    return Product::productID;
}

int Product::getNumberSold() {
    return Product::numSold;
}

double Product::getTotalPaid() {
    if(Product::totalPaid < 1) {
        totalPaid = 0;
    }
    return Product::totalPaid;
}

int Product::getInventoryCount() {
    Product::inventoryCount = 0; 
    return Product::inventoryCount;
}

void addShipment(int shipmentQuantity, double shipmentCost) {
    Product::inventoryCount = Product::inventoryCount + shipmentQuantity;
    Product::totalPaid = Product::totalPaid + shipmentCost;
}

void reduceInventory(int purchaseQuantity) {
    Product::inventoryCount = Product::inventoryCount - purchaseQuantity;
    Product::numSold = Product::numSold + purchaseQuantity;
}

double getPrice() {
    int price = (Product::totalPaid / static_cast<double>(Product::inventoryCount + Product::numSold)) * 1.25;
    return price;
}