Add-on: Inventory Specials.
An item special gives a set price when a buyer gets a set number of items.
This is different from a coupon code.

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Set a price for buying a set number of items.
Add-on: Inventory Specials.
An item special gives a set price when a buyer gets a set number of items.
This is different from a coupon code.
Edit the item in Inventory and open Item Specials. Set the number of items needed and Special total price. Save the item.
Use a Special category for mix & match items only when the items should share the deal. Check that the prices and rules match across that group.
Add the needed items on Sell. When the deal applies, check the special line and price in the cart.
Check the shop's cart before advertising an online offer, since a POS special may not apply there. Use the Coupons add-on for shop coupon offers.