The terms are simple.
I create & sell custom modules for OpenCart at extremely fair prices. My modules are guaranteed to work with the default OpenCart installs that have corresponding version listings.
I make no guarantees that they will work with any customizations you may have already made to your cart.
There are obviously no refunds for any code purchased.
As far as why I charge for modules for an "open source" platform... I offer my code modules to work with OpenCart's code base. Code is like any other product. You use an open source cart to sell your "products". Just as I am using OpenCart to sell my "products". It also ensures my guarantee on working code. I always keep my modules up to date with the latest version as soon as I can.
Support of future OpenCart versions
I intend to always keep my addons up to date with the latest versions of the core software. However, in the case where an addon is rendered unnecessary because of a similar core feature or when a module turns out to be more work to maintain than it is worth, I may choose to discontinue support for it. These are rare and unlikely conditions. Typically in the case of a discontinued item, a viable replacement is created, and this replacement may or may not be available to previous purchasers of the discontinued mod for a discounted price. In most cases, upgrades to future versions are included with the price of the initial purchase.
Most of my addons are designed to be backward compatible with the past few major versions. For example (1.3.x and 1.4.x). When a new major version is released, that typically means i will be dropping support for the older version. It is recommended that you try to keep your store updated whenever possible. OpenCart upgrading is getting easier now and it should be safer to upgrade in many cases, at least between minor versions (e.g 1.4.8 to 1.4.9).