Checkbox an values to transmit to database - php

Question

I'm new here and I need help with a PHP code.
I want to transform a radio button into a checkbox, make it calculate the fee depending on how much checkboxes are checked, and to transmit the checked values to the predefined database.
My code only takes the one predefined value for fee an doesn't actually adds them up when I click on the other checkbox. I tried everything but with no success. Here is the original unchanged code:
<script src="<?php echo URL_FRONT_JS;?>jquery.js"></script>
<script>
function get_tutor_course_details()
{
course_slug = $('#course_slug option:selected').val();
selected_date = $('#start_date').val();
if(!course_slug || !selected_date) {
$('#fee').text('');
$('#duration').text('');
$('#days_off').text('');
$('#content_li').remove();
$('#time_slot_div').text('<?php echo get_languageword("please_select_course_and_date_first"); ?>');
return;
}
$.ajax({
type: "POST",
url: "<?php echo URL_HOME_AJAX_GET_TUTOR_COURSE_DETAILS; ?>",
data: { "course_slug" : course_slug, "tutor_id" : <?php echo $row->id; ?>, "selected_date" : selected_date },
cache: false,
beforeSend: function() {
$('#time_slot_div').html('<font color="#5bc0de" size="6"> Loading...</font>');
},
success: function(response) {
if(response == "") {
$('#fee').text('');
$('#duration').text('');
$('#days_off').text('');
$('#content_li').remove();
$('#time_slot_div').html('<?php echo get_languageword("no_slots_available."); ?> <?php echo get_languageword("click_here_to_send_me_your_message"); ?>');
$('#request_tutor_btn').slideUp();
} else {
var fee_duration = response.split('~');
var fee = fee_duration[0];
var duration = fee_duration[1];
var content = fee_duration[2];
var time_slots = fee_duration[3];
var days_off = fee_duration[4];
$('#fee').text(fee +'€');
$('#duration').text('EUR');
if(content) {
$('#content_li').remove();
$('#course_li').after('<li id="content_li"><?php echo get_languageword("course_content"); ?><p>'+content+'</p></li>');
}
time_slot_html = "";
if(time_slots != "")
time_slots = time_slots.split(',');
total_available_timeslots = time_slots.length;
if(total_available_timeslots > 0) {
for(i=0;i<total_available_timeslots;i++) {
check_radio = "";
if(i == 0)
check_radio = 'checked = "checked"';
time_slot_html += '<li><div><input id="radio1'+i+'" type="radio" name="time_slot" value="'+time_slots[i]+'" '+check_radio+' ><label for="radio1'+i+'"><span><span></span></span>'+time_slots[i]+'</label></div></li>';
}
$('#time_slot_div').html(time_slot_html);
$('#request_tutor_btn').slideDown();
} else {
$('#time_slot_div').html('<?php echo get_languageword("no_slots_available."); ?> <?php echo get_languageword("click_here_to_send_me_your_message"); ?>');
$('#request_tutor_btn').slideUp();
}
}
}
});
}