If you want to send your client a reminder for an outstanding invoice, then this can be done very simply by using the carbon copy email. The carbon copy email is an exact copy of the email that your client receives, containing your invoice PDF attachment. It is always simultaneously sent to your own email address, whenever you send an invoice to someone through the app. Please note: this carbon copy email will always go to the email that you used to register your Albert account with.
Simply forward this email on to your client and attach a message which highlights your purpose, to avoid confusion. Meaning, if your intention is for it to be a receipt, then specify this within the subject line or within the content of your message within the email. You also have the bonus option of being able to add recipients and/or attach files, if need be.