I am attempting to complete the phone bill calculator for my C++ course. I have been provided the following:
Plan A:
Price: $39.99 per month.
Data allowance: Includes 4 gigabytes of data. Additional data costs $10 per gigabyte.
Plan B:
Price: $59.99 per month.
Data Allowance: Includes 8 gigabytes of data. Additional data costs $5 per gigabyte.
Plan C:
Price: $69.99 per month.
Data Allowance: Unlimited
I feel I am improperly implementing the switch cases as well as the if statements. On top of that, if the user inputs a non-numeric value for gigsUsed, the program displays the total based on the first else if statement. Example: user inputs “B” for gigsUsed. The output will show “The total amount due is $39.99. How can I make sure only valid numeric inputs will be taken? I apologize in advance for any silly errors being made both with my code and with posting this question, I am new to programming, and this is my first post to this site.

// Display menu & prompt user for plan type (1-3) and gigabytes of data used (not a negative value)
cout << "nSelect a subscription package:" << endl
    << "t1. Package A" << endl
    << "t2. Package B" << endl
    << "t3. Package C" << endl
    << "t4. Quit" << endl
    << "Package: ";
cin >> plan;

// Drive menu options
switch (plan)
{
case 1: // Plan A
    // Prompt User for total amount of gigabytes used
    cout << "nHow many gigabytes of data were used? ";
    cin >> gigsUsed;

    // Determine total amount due
    if (gigsUsed > 4)
        totalOwed = (gigsUsed - 4) * 10 + 39.99;
    else if (gigsUsed <= 4)
        totalOwed = 39.99;
    else if (gigsUsed < 0)
        cout << "nError ... Invalid gigabytes entered. Try Again." << endl;
    

    // Display total amount due
    cout << "nThe total amount due is $" << totalOwed << endl;

    break;

case 2: // Plan B
    // Prompt User for total amount of gigabytes used
    cout << "nHow many gigabytes of data were used? ";
    cin >> gigsUsed;

    // Determine total amount due
    if (gigsUsed > 8)
        totalOwed = (gigsUsed - 8) * 5 + 59.99;
    else if (gigsUsed <= 8)
        totalOwed = 59.99;
    else if (gigsUsed < 0)
        cout << "nError ... Invalid gigabytes entered. Try Again." << endl;

    // Display total amount due
    cout << "nThe total amount due is $" << totalOwed << endl;

    break;

case 3: // Plan C
    // Display total amount due
    totalOwed = 69.99;
    cout << "nThe total amount due is $" << totalOwed << endl;

    break;

case 4: // Exit option
    // Let the user know they are exiting the program
    cout << "nYou have chosen to exit the program ... Goodbye!" << endl;

    break;

default: // Error
    cout << "Error ... Invalid Package. Try again." << endl;
    break;
}

return 0;

}