Having trouble printing out my 'receipt' on my C++ invoice program: Function for some reason not working

Question

Trying to wrap up my invoice program. I'm trying to get my customerInvoice function to properly display. However whenever I try to use it, I get an error. For some reason this function is having a difficult time displaying, and I'm not sure what the complication is. I can enter my values in just fine, none of the other variables are having a hang-up, I'm just getting a bit stalled on this part as its not properly displaying the message inside the customerInvoice function.

I also want my invoice to recognize the senior discount: if the customers age is over 60, then they get the 0.05 multiplier discount on their purchase.

#include <iostream>
#include <iomanip>
#include <string>
using namespace std;


const float flatTaxRate = 0.08;
const float seniorDiscountRate = 0.05;

int getIntValue (string toInput){
    cout << "Please enter " << toInput <<": ";
    int value;
    cin >> value;
    return value;
}

string choiceOfItem(int choice) {
    return choice == 1 ? "Bread" :
           choice == 2 ? "Milk" :
           choice == 3 ? "Soap" :
           choice == 4 ? "Eggs" :
           choice == 5 ? "Deodorant" :
           choice == 6 ? "Juice" :
           choice == 7 ? "Chips" :
           choice == 8 ? "Forks" :
           choice == 9 ? "Spoons" :
           choice == 10 ? "Cups" :
    "Invalid Item: Select Again";
}

int getIntInputValues (string toInput){
    cout << "Please enter " << toInput <<": ";
    int value;
    cin >> value;
    return value;
}

double getDoubleInputValues (string toInput){
    cout << "Please enter " << toInput <<": ";
    double value;
    cin >> value;
    return value;
}

bool customerAge(int age){
    const int seniorCustomer = 60;
    return age >= seniorCustomer;
}

void customerInvoice(int choice, double price, int age)
{
    cout << "-----------------------------------\n";
    cout << "\n" << "Invoice" << "\n";
    //can use \n as a tab
    
    if(seniorCustomer(age))
    {
        price -= price * seniorDiscountRate;
    }
     
    cout << choiceOfItem(choice) <<":" << "\n" <<"$"<< price << endl;
    cout << "Tax:" << "\n" <<"$"<< price * flatTaxRate << endl;
    cout << "Discount:" << "\n" <<"$"<< "-" << price * seniorDiscountRate << endl;
    cout << "Total:" << "\n"<< "$" << price + (price * flatTaxRate) << endl;
    cout << "-----------------------------------\n";
}
 



int main()
{
    const int MIN_VALID = 1;
    const int MAX_VALID = 10;

    cout << "What would you like to buy?" << endl;
    
    for (int choice = MIN_VALID; choice <=MAX_VALID; ++choice)
    {
        cout << choice << ". " << choiceOfItem(choice)<< endl;
    }
    cout << endl;    

    int choice = getIntValue("your choice");

   
    while (choice < MIN_VALID || choice > MAX_VALID)
    {
        cout << "Sorry, " << choice << " wasn't a valid choice" << endl;
        cout << "Please try again:" << endl;
        choice = getIntValue("your choice");
    }
    
    double price = getDoubleInputValues("price");

    
    int age = getIntInputValues("age");

       
    bool seniorCustomer;
       
    cout << " " << endl;
        
    customerInvoice;
    
    cout << choiceOfItem(choice) << endl;
    cout << price << endl;
    cout << age << endl;
    cout << seniorCustomer;
    
    return 0;
}

Answer 1

As noted in comments, the following in main does not equate to calling a function:

customerInvoice;

Rather you want:

customerInvoice(choice, price, age);

Answer 2

In the main() function, you have this statement:

customerInvoice;

customerInvoice is name of function and a function name is an expression, here it's result is unused. When compiling your program, compiler is giving this warning:

warning: expression result unused [-Wunused-value]
customerInvoice;
^~~~~~~~~~~~~~~

Instead, call customerInvoice() function and pass the appropriate arguments to it:

customerInvoice(choice, price, age);

> I also want my invoice to recognize the senior discount: if the customers age is over 60, then they get the 0.05 multiplier discount on their purchase.

Looks like you need to call customerAge() instead of seniorCustomer() function in customerInvoice() function. Moreover, there is no seniorCustomer() function in your program.
It would be much better if you rename your customerAge() function to seniorCustomer() as it is returning whether customer is a senior customer or not.