<input name="shipping_method" value="dif.dif" id="dif.dif" checked="checked" type="radio">
if($('#dif.dif').prop('checked')) {
$('#buttons').hide();
}
Naming rules:https://www.w3schools.com/tags/att_global_id.asp
Must contain at least one character
Must not contain any space characters
In HTML, all values are case-insensitive
$('[id="dif.dif"').css('outline', 'dashed 2px red')
$('#dif\\.bar').css('outline', 'dashed 2px blue')
https://jsfiddle.net/n53naxsm/