Methods) Ultimate Keychains for sale

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        System.out.println("Ye Olde Keychain Shoppe");

        int numOfKeychains = 0 ;
        int price = 10 ;
        double salesTax = 8.25 ;
        double shippingCost = 5.00 ;
        double perKeychainShipping = 1.00 ;

        while(true) {
            System.out.println("\n1. Add Keychains to Order");
            System.out.println("2. Remove Keychains from Order");
            System.out.println("3. View current Order");
            System.out.println("4. Checkout");

            System.out.print("Please enter your choice: ");
            Scanner keyboard = new Scanner(System.in);
            Integer input = keyboard.nextInt();

            if (input == 1) {
                numOfKeychains = add_keychains(numOfKeychains);
                System.out.println("\nYou now have " + numOfKeychains + " keychains.");

            } else if (input == 2) {
                numOfKeychains = remove_keychains(numOfKeychains);
                System.out.println("\nYou now have " + numOfKeychains + " keychains.");

            } else if (input == 3) {
                view_order(numOfKeychains, price, salesTax, shippingCost, perKeychainShipping);
            } else  if (input == 4){
                checkout(numOfKeychains, price, salesTax, shippingCost, perKeychainShipping);
                break;
            } else {
                System.out.println("\nIncorrect menu choice, please try again");
            }
        }
    }

    public static int add_keychains (int numOfKeychains){
        while (true) {
            System.out.print("\nYou have " + numOfKeychains + " keychains. How many to add? ");
            Scanner keyboard = new Scanner(System.in);
            Integer numToAdd = keyboard.nextInt();

            if (numToAdd > 0) {
                return (numOfKeychains + numToAdd);
            } else {
                System.out.println("Please enter a positive number of Keychains to add.");
            }
        }

    }

    public static int remove_keychains (int numOfKeychains){
        while (true) {
            System.out.print("\nYou have " + numOfKeychains + " keychains. How many to remove? ");
            Scanner keyboard = new Scanner(System.in);
            Integer numToRemove = keyboard.nextInt();

            int totalKeychains = (numOfKeychains - numToRemove);
            if (totalKeychains > 0) {
                return totalKeychains;
            } else {
                System.out.println("You may not have a negative total number of Keychains, please try again.");
            }
        }
    }

    public static void view_order (int numOfKeychains, int price, double salesTax , double shippingCost , double perKeychainShipping){
        System.out.println("\nYou have " + numOfKeychains + " keychains.");
        System.out.println("Keychains cost  " + price + " each (without tax).");
        System.out.println("Tax rate is " + salesTax + ".");
        System.out.println("The shipping cost per item is $" + perKeychainShipping + " plus a base rate of $" + shippingCost + ".");

        System.out.println("Total cost is $" + ((numOfKeychains * price * (100 + salesTax)/100) + (numOfKeychains * perKeychainShipping) + shippingCost ) + ".");
    }

    public static void checkout (int numOfKeychains, int price, double salesTax , double shippingCost , double perKeychainShipping){
        System.out.print("\nWhat is your name?: ");
        Scanner keyboard = new Scanner(System.in);
        String name = keyboard.next();
        view_order(numOfKeychains , price , salesTax , shippingCost , perKeychainShipping);
        System.out.println("Thanks for your order, " + name + ".");

    }
}