Well for starters, with PHP forms like this you always have to do the HTML end of things first:
the function I am interested in is the use of the radial buttons and the way it tallies the price
submit.php - processes the information
<form action="submit.php" method="post">
<input type="checkbox" name="checkbox1" value="15">
<input type="checkbox" name="checkbox2" value="45">
Let's break down the above script:
$field1 = $_POST["checkbox1"];
$field2 = $_POST["checkbox2"];
$output = $field1 + $field2;
first line: "$" always starts before defining a variable, which in this case it's contents is checkbox1's value.
Line two is the same except your defining the second checkbox.
Line four processes the information, adding $field1 and $field2.
Line five is a simple statement "echo" which writes the variable $output.
The outcome should be "60"
That should point you in some direction.