You could create a Joomla menu item to CBSubs invoice page then provide that link in your emails. To create such a menu item set the menutype to Community Builder > Plugin and for Plugin select CB Paid Subscriptions. If wherever you're sending the email from has access to the invoice id you could also use the below to provide a URL directly to the invoice, but I don't think any of the emails in CBSubs have access to the invoice id.
Code:
index.php?option=com_comprofiler&view=pluginclass&user=USER_ID_HERE&plugin=cbpaidsubscriptions&cbpact=showinvoice&cbpinvoice=INVOICE_ID_HERE