The key to sending multiple records/certificates with an invoice in one email is to issue the certificate first, but not email them.
Issue the invoice last, and email it, and all the issued records/certificates will attach to this email, including the invoice.
On the website, after producing a form for a customer, you will be able to click 'Approve and Print' instead of the 'Approve and Email' option. This will generate a PDF of the certificate without emailing it to the customer and print it to your screen (not your paper printer).
Alternatively you can click 'Approve and Email' and on the next screen, instead of clicking 'Save', click 'Cancel'. The certificate will still be issued but it won't be emailed from the software, just the same as clicking 'Approve and Print'.
Afterwards, when you go to produce your next form, be sure it's produced under the same job as the last and then proceed to do the same, click 'Approve and Print'.
Once you're done and you're ready to produce your invoice, this time once your invoice is completed be sure to click 'Approve and Email', rather than the 'Approve and Print'. This will take you to a new page which shows the template being used for the email that's about to be sent, the invoice, as well as the attachments that will be sent with the email.
On the app, you would first need to produce a certificate and when you get to the last stage, after issuing the certificate, when it prompts you to email it to the customer, instead of pressing "Send email", press "Home".
Now if you click New Estimate, Quote or Invoice > New Invoice > Existing Job and select the job that is associated with the above form you have created. If you then complete the invoice and press "Issue" it will take you to the email screen and under attachments there will be both the original form and the invoice. Then when you click "Send Email" it will send out the email with both the attachments.