Calculate savings from subscribe-and-save auto delivery.
Subscribe-and-save programmes cut a percentage off the shelf price and usually waive shipping. The real saving is the gap between what a year of one-off orders would cost (price plus shipping each time) and what a year of discounted auto-shipments costs. Expressing that gap as a percentage of the one-off cost gives the effective discount, which is normally larger than the headline percentage because shipping disappears too.
Auto-ship saving
Yearly savings = (regular price + shipping) x deliveries - regular price x (1 - discount/100) x deliveries
Usually it understates it. Waived shipping adds to the discount, so the effective saving shown here is higher than the advertised percentage whenever shipping would otherwise be charged.
Lower the deliveries-per-year figure to what you will actually consume. Auto-ship stops being a saving the moment you are stockpiling product you never use.
Most retailers allow it, but some claw the discount back or require a minimum number of shipments. Check the terms before treating the first-order discount as free money.