A quick but important update on this.
1) The Online system for the appointment is flexible enough.
2) IF YOU ARE MARRIED TO A PERUVIAN AND YOU ARE DOING THE PRORROGA FAMILIAR - YOU DO NOT NEED TO PAY THE $50, ONLY THE 25.6 SOLES. EVEN THOUGH THE MIGRACIONES WEBSITE TELLS YOU THAT YOU NEED TO PAY THIS. My bolete was returned and I was told as I was married to a Peruvian I should go claim a refund - which I did.