I'd like to run a penny sale, where the customer purchases 3 bottles of a product and receives a 4th one for a penny. What is the best way to do this?
If the sale is on a specific product, then your best bet is probably to set up an auto-add promo so that the 4th bottle for a penny simply shows up in the cart once three of that product are added.
First set your auto-add preferences (Products/Preferences). There you would override the product visibility and select "Locked to Trigger" as your Quantity relationship. If you want people to get a penny bottle for every three bottles they buy, then select Multiply Quantity.
Then you would create the penny product in the OMS, with visibility internal, and set up the auto-add on that product. To do so, select 1 auto-add on page 1, then on page two, set auto add qty of 1, trigger type product, trigger qty 3, and select the triggering product.
Then test the heck out of it to make sure it's working in accordance with your wishes :-)
PS- this auto-add functionality is very powerful. We suggest that as you set this up, you read the ? tooltips in the OMS along the way.