Checkbox's should add $15 each to the total of a payment form

I have created a form which collects participant information and payment for an event. There are optional extras in the form, and each should add $15 to the total.

Site is built on Adobe Business Catalyst if you are wondering about the odd ID's

Checkboxes:

    <label for="CAT_Custom_382934_296996">Early Bird (8am Dropoff with Breakfast) 
       <strong>+ $15</strong>
    </label>
    <input id="CAT_Custom_382934_296996" name="CAT_Custom_382934_296996" type="checkbox" value="1">

    <label for="CAT_Custom_382935_296996">Extra Time (7pm Pickup with Dinner)     
      <strong>+ $15</strong>
    </label>
    <input id="CAT_Custom_382935_296996" name="CAT_Custom_382935_296996" type="checkbox" value="1">

Then if selected will add $15 each to the total.

<input class="cat_textbox" id="Amount" name="Amount" readonly="readonly" type="text" value="40">

Please tell me how I can have the selected checkboxes add to the total.

Any help appreciated. Thank You.

Screenshot of the problem