This is an extremely effective way to keep customers loyal to you and get many orders that you may be missing out on.
What happens is when a customer purchases a product you can put in when you expect them to run out of the product. The customer history or your experience with similar customers should be able to give you an idea. For example, a large dog food parcel generally lasts three months so say you put an automatic note to send an email to the customer in two months and a bit. Just before you expect it to run out. Similarly a box of pens you may expect will last them two weeks, so you would set the order for say 10 days.
To date, we see about a 35% open rate automated customer replenishment ordering.
This shows the importance of a VIP system.