Override this page https://github.com/bagisto/bagisto/blob/master/packages/Webkul/Shop/src/Resources/views/checkout/onepage.blade.php and here is the submit button https://github.com/bagisto/bagisto/blob/master/packages/Webkul/Shop/src/Resources/views/checkout/onepage.blade.php#L90

add this crossDomain: true, in your js code under ajax.

You can change the return URL from here https://github.com/bagisto/bagisto/blob/master/packages/Webkul/Paypal/src/Payment/Standard.php#L50

what will you get when your print the $order ?

Please share your instance show that I can look into it.

you have to pass the string, you are sending object as a parameter.

If you want popup in the same page then you have to add your js ,

Create a blade file
put this code into it
@if(request()->is('checkout/onepage'))
eventBus.$on('after-checkout-payment-section-added', function() {
$(document).ready(function() {
// your code here
});
});

@endif

Please share your instance and ftp details, so that we will assist you in better way.

Try to to check what you are getting in the post to do so, write dd(request()->all()); in your code.

Try using get method or you can also refer paytab documentation https://dev.paytabs.com/