You can easily download your payment invoice by accessing your doctorSIM account control panel. In the "Unlocks and Checks" section, you'll be able to download your invoices.
Remember, you need to have a doctorSIM account to download an invoice. You can create your account here, it takes no time at all, and of course, it's free.
Additionally, to view your invoice, you should have previously completed your billing information in the "Info and Settings" section.