Set the offer once. The box on your product page and the discount at checkout read the same offer, so the price a customer sees is the price they pay, down to the cent. Three kinds, each its own block: quantity tiers, a fixed bundle and a free gift. Each inherits your theme's colours, type and corners instead of bringing its own. The discount follows the quantity in the cart, not a click on the box. Type it by hand and the price is the same. One stylesheet, one small script, no tracking.
Add Product Add-ons, Post Purchase & Checkout Upsell...
Sell More & Increase Your Average Order with Bundles...
Increase average order value with Mix and Match bundle offers...
Mix and match bundles, quantity breaks, volume discounts