<form action="mailto:you@yourdmainhere.com" method="post" enctype="text/plain" >
FirstName:<input type="text" name="FirstName">
Email:<input type="text" name="Email">
<input type="submit" name="submit" value="Submit">
</form>

ref. https://mailtrap.io/blog/javascript-send-email/

shopping cart checkout ใน 1 หน้า
ref. https://phppot.com/php/one-page-checkout-script-free-with-example-template-in-php/