If you want to get a copy of a recent invoice you can do so within Timetastic itself.
If you're an Admin user, you can just login and select:
SETTINGS > BILLING
and scroll down the page.
You'll see copies of all your previous invoices / receipts are available here to download.
If you don't see SETTINGS, you'll need to contact one of your Timetastic Admin users to download invoices.
It's also worth noting that you can set two unique email addresses to receive a copy of the monthly invoice, it's covered in the help centre here.