This can be done but there are a lot of variables involved. (If it was a simple mod, I’d package it up as a simple mod!) So I offer it as a service. It’s called Free Shipping Recalculation. It recomputes your free shipping threshold based on the subtotal after discounts.
Free Shipping Recalculation works, whether you are using Admin->Modules->Shipping->Free Shipping Options or Admin->Modules->Order Totals>Shipping to set your free shipping threshold.
This video demonstrates the creation of product packages (groups of individual products) that can be sold as a discounted group with a single click in Zen
Cart, using my mods Discount Chooser and Extended Family.
For a lot of stores, it makes sense to build “product bundles” that can be sold with a single click. As long as your products don’t have attributes, you can easily do this with Zen Cart Extended Family.
Discount Chooser simplifies the process of creating discounts based on items in the cart. You can create buy one, get one at a discount, buy two get one at a discount, or buy get a choice of at a discount. Item specification may be done by category or by product.
If you want offers involving more than just two items, Discount Chooser is a better option for your OpenCart based store than Better Together. An example of this would be "buy two, get one free."
Better Together Admin allows you to easily create “buy one, get one” discounts for your OpenCart based store. And it works right through your regular admin panel; no need to edit code to create the discounts. Here’s a video:
I have released Discount Chooser for OpenCart (a port of the Discount Chooser / Free Gift Chooser algorithm from Zen Cart). I’m excited about this software and hope it really helps OpenCart store operators.
Discount Chooser goes beyond the standard BOGO discounts to permit the creation of much broader offers. For example, you can create “Buy 2, get 1”, “Buy 3 get 1,” or “Buy some number, get a choice of some number.”