Solution:
Here is my solution. Use jQuery/Javascript. I advise adding the id=”donation” to the donation input field, and also a placeholder=”10.00″ as an example of donation input.
jQuery(document).ready(function($) {
var first_price = 0;
var second_price = 0;
var total_price = 0;
$('#registration_fee').on( "change", function() {
first_price = $('#registration_fee').val();
first_price = parseInt( first_price );
//console.log( first_price );
});
$('#donation').on( "change", function() {
second_price = $('#donation').val();
second_price = parseInt( second_price );
//console.log( second_price );
});
$('#payment-form').on( "change", function() {
total_price = first_price + second_price;
//console.log( "total: " + total_price );
$('.stripe-button-el span').html( "Pay with Card $" + total_price );
});
});