CTRL+F and look for this part of the code:
var total = (devamount.val()*1) + (txfee.val()*1);
Edit it to:
var total = (txfee.val()*1);
Optionally, if you want to avoid any other JS errors and make the code cleaner, also remove this part:
var devamount = $("#developerDonation");
if((!isNaN(devamount.val())) && devamount.val()>=0){
$(devamount).parent().removeClass('has-error');
} else {
$(devamount).parent().addClass('has-error')
}