checkbox value 0 or 1

Thanks worked as in

$('#custom7').on('change', function(){
   this.value = this.checked ? 1 : 0;
   // alert(this.value);
}).change();

link: http://jsfiddle.net/WhQaR/

It makes no much sense because unchecked checkboxes are not sent to the server as said in the comments, but if you need a value in the server you can use a hidden field:

HTML:

<input type="checkbox" value="1" id="custom7" checked="checked" /> 
<input type="hidden" value="1" id="hdncustom7" name="custom7" />
<label for="custom7">Email me more info?</label>

jQuery:

$('#custom7').on('change', function(){
   $('#hdncustom7').val(this.checked ? 1 : 0);
});

Using this methods you will receive custom7 with 0 or 1 in the server

Don't forget about the bitwise XOR operator:

$('input[type="checkbox"]').on('change', function(){
    this.value ^= 1;
});

$('input[type="checkbox"]').on('change', function(){
    this.value ^= 1;
    console.log( this.value )
});

<label><input type="checkbox" name="accept" value="1" checked> I accept </label>
<label><input type="checkbox" name="accept" value="0"> Unchecked example</label>

<script src="//code.jquery.com/jquery-3.3.1.min.js"></script>

Just to be a butt and offer a slightly shorter answer:

$('input[type="checkbox"]').change(function(){
    this.value = (Number(this.checked));
});