Version 3.2
<p>
<span style="font-size:12px;"><span style="font-family:verdana,geneva,sans-serif;">Hi {contact.first_name},<br />
<br />
An invoice has been created for your account and is attached to this email in PDF format.<br />
{% for invoice in invoices %}<br />
Invoice #:<strong> {invoice.id_code}</strong></span></span><br />
<span style="font-family: verdana, geneva, sans-serif;">{% endfor %}</span></p>
<p>
<span style="font-size:12px;"><span style="font-family: verdana, geneva, sans-serif;"><a href="https://{invoice.payment_url}">Pay now</a> (log in not required)</span></span></p>
<p>
<span style="font-size:12px;"><span style="font-family: verdana, geneva, sans-serif;">If you have any questions about your invoice, please let us know!</span></span></p>
Apologies for the horrible markup.
Output is a link to "https://" and no actual URL.
The invoice or invoice.payment_url isn't mentioned in the "Available Tags" section so initially i assumed it wasn't possible.