BuyNow button If terms and conditions checked

Have a look at plugins/paypal-express-checkout/views/frontendcode.php
You can add a checkbox there plus a small javascript that check if ticked when submit is enabled.
Code is currently as follows:

<form method="post" action="<?php echo $config->getItem('plugin_form_handler_url'); ?>">
  <input type="hidden" name="AMT" value="<?php echo $atts['amount']; ?>" />
  <input type="hidden" name="CURRENCYCODE" value="<?php echo $atts['currency']; ?>" />
  <?php if ( isset($atts['description']) ) { ?>
    <input type="hidden" name="PAYMENTREQUEST_0_DESC" value="<?php echo $atts['description']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['tax']) ) { ?>
    <input type="hidden" name="TAXAMT" value="<?php echo $atts['tax']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['shipping']) ) { ?>
    <input type="hidden" name="SHIPPINGAMT" value="<?php echo $atts['shipping']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['handling']) ) { ?>
    <input type="hidden" name="HANDLINGAMT" value="<?php echo $atts['handling']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['qty']) ) { ?>
    <input type="hidden" name="PAYMENTREQUEST_0_QTY" value="<?php echo $atts['qty']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['return_url']) ) { ?>
    <input type="hidden" name="RETURN_URL" value="<?php echo $atts['return_url']; ?>" />
  <?php } ?>

  <?php if ( isset($atts['cancel_url']) ) { ?>
    <input type="hidden" name="CANCEL_URL" value="<?php echo $atts['cancel_url']; ?>" />
  <?php } ?>

  <input type="hidden" name="func" value="start" />

  <?php if ( isset($atts['button_style']) ) { ?>
    <?php if ( $atts['button_style'] == 'buy_now' ) { ?>
      <input type="image" value="" src="<?php echo $config->getItem('buy_now_button_src'); ?>" />
    <?php } elseif ( $atts['button_style'] == 'checkout' ) { ?>
      <input type="image" value="" src="<?php echo $config->getItem('checkout_button_src'); ?>" />
    <?php } ?>
  <?php } else { ?>
    <input type="submit" value="Pay with PayPal" />
  <?php } ?>
</form>

Add somewhere :

<input type="checkbox" name="terms" value="agree">Agree terms and conditions<br>

And add onsubmit=”return checkifagree()” also give it a name so the form header looks like this:

<form name="paypal_form" method="post" action="<?php echo $config->getItem('plugin_form_handler_url'); ?> " onsubmit="return checkifagree()">

finally define checkifagree() function and do the checks.

function validateimeiWidget()
{
var checkbox=document.forms["paypal_form"]["terms"].value;
   if( checkbox == ''agree")
       {
           return true;
      }
return false;

}

I am not a programmer; please test and bespoke the code as you wish; this is just to give you an idea. If you managed to get it to work please share your code ??

sorry I pasted the wrong function name:
function validateimeiWidget()
should be
function checkifagree()