Try this one…
export function button48_click(event) {
let vouchers = []
//-----------------------------------
vouchers[0] = $w('#input80').value;
vouchers[1] = $w('#input81').value;
vouchers[2] = $w('#input82').value;
vouchers[3] = $w('#input83').value;
//-----------------------------------
if(vouchers[0] === undefined) {vouchers[0] = "0";}
if(vouchers[1] === undefined) {vouchers[1] = "0";}
if(vouchers[2] === undefined) {vouchers[2] = "0";}
if(vouchers[3] === undefined) {vouchers[3] = "0";}
//-----------------------------------
setTimeout(()=>{console.log(vouchers.length);
console.log(vouchers);
$w('#html1').postMessage(vouchers);
},100);
}
<html>
<head>
<script type="text/javascript">
function init(){
window.onmessage = (event) => {console.log(event)
let receivedData = event.data; console.log(receivedData)
document.getElementById("md").value = receivedData[0],
document.getElementById("fd").value = receivedData[1],
document.getElementById("mc").value = receivedData[2],
document.getElementById("fc").value = receivedData[3],
}
}
</script>
</head>
<body onload="init();">
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="charset" value="utf-8">
<input type="hidden" name="lc" value="en_US">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="business" value="**redacted**">
<input type="hidden" name="upload" value="1">
<!-- item 1 -->
<input type="hidden" name="item_name_1" value="Male Dog Voucher">
<input type="text" id="md" name="quantity_1" value="">
<input type="hidden" name="amount_1" value="65.00">
<!-- item 2 -->
<input type="hidden" name="item_name_2" value="Female Dog Voucher">
<input type="hidden" id="fd" name="quantity_2" value="">
<input type="hidden" name="amount_2" value="90.00">
<!-- item 3 -->
<input type="hidden" name="item_name_3" value="Male Cat Voucher">
<input type="hidden" id="mc" name="quantity_3" value="">
<input type="hidden" name="amount_3" value="25.00">
<!-- item 4 -->
<input type="hidden" name="item_name_4" value="Female Cat Voucher">
<input type="hidden" id="fc" name="quantity_4" value="">
<input type="hidden" name="amount_4" value="45.00">
<input type="image" src="https://static.wixstatic.com/media/e1d3bb_b5890e554cb0453cb453e94db8a858f4~mv2.png" border="0" height="57" width="263" name="submit" upload="1" id="btn" onclick="button_click" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
</body>
</html>