C# checkBox Logic .NET

Since the AND condition is more restrictive than the others, you should check it first. Like this:

private double Oillube()
    {
        //if they are both checked
        if(lubeChbox.Checked && oilChbox.Checked)
        {
            //Creating Variables sum to hold the value of the two
            double sum;

            sum = OIL_CHANGE + LUBE_JOB;

            //returning sum
            return sum;

        }
        //checking if the Oil is checked
        if (oilChbox.Checked)
        {
            return OIL_CHANGE;
        }

        //checking if the lube is checked
        if (lubeChbox.Checked)
        {
            return LUBE_JOB;
        }

            //just returning to get ride off the red line under the Method
            return 0;
    }

The lubeChbox.Checked && oilChbox.Checked condition needs to go first, as it is the more specific of the two.

The way you have it now if either lubeChbox.Checked is true, or oilChbox.Checked is true it will return before reaching the lubeChbox.Checked && oilChbox.Checked condition, meaning there is no situation in which it could ever reach the condition unless both were false.

This is why it is typically a bad idea for people to use return in this manner unless they fully understand the flow of the code they want to achieve.

Here's the corrected code:

private double Oillube()
{
    //if they are both checked
    if(lubeChbox.Checked && oilChbox.Checked)
    {
        //Creating Variables sum to hold the value of the two
        double sum;

        sum = OIL_CHANGE + LUBE_JOB;

        //returning sum
        return sum;

    }

    //checking if the Oil is checked
    if (oilChbox.Checked)
    {
        return OIL_CHANGE;
    }

    //checking if the lube is checked
    if (lubeChbox.Checked)
    {
        return LUBE_JOB;
    }

    //just returning to get ride off the red line under the Method
    return 0;
}

Alternatively you can use tertiary operators for this situation like so:

private double Oillube()
{
    return (lubeChbox.Checked ? LUBE_JOB : 0) + (oilChbox.Checked ? OIL_CHANGE : 0);
}

A tertiary operator works the same as an if statement only they work inline.

So this:

(lubeChbox.Checked ? LUBE_JOB : 0)

is equivalent to this pseudo-code:

if(lubeChbox.Checked) { LUBE_JOB } else { 0 }

Dotnetfiddle example

It's because if the first checkbox is checked, then first if statement is true, so you are returning value, thus ending the method.

You should replace your if statements, last one should become first, as it is most restrictive.