To e-mail remittance advice (payment receipts) just go to Sales > Select the Invoice > Scroll down to the bottom and under the payment details section click the envelope icon.  You'll then be prompted to enter in the customers e-mail address (this automatically fills if you've got one set in the customers profile), a subject (this automatically fills with 'Payment Receipt from [your company name]), message text.  When you're ready just press the  Send Email button.