Have you tried the link that was sent to the client in that notice? If it doesn't take them to the pay page, there may be an issue with the company hostname, encryption key (unlikely), or routing customization that causes Blesta to redirect somewhere else or otherwise deem the payment link invalid. Make sure you're using the {payment_url} tag.
The notice emails don't generate and send off PDF invoice attachments to the email. That might be a nice feature request though.
Since the notice email supports the {invoice} that is late, you should be able to reference the amount due on the invoice with the {invoice.due} tag. Of course, you'll want it to be formatted nicely in the currency, so you can set a filter as I described here.
e.g.
Amount due: {invoice.due | currency_format invoice.currency}
The output in the email would then be something like:
Amount due: $56.25