I have been creating modules for OpenCart 1.x since it was first released, so I have quite a few. Sometimes others make similar modules as well on their own accord. This results in multiple modules that do the same thing more or less. There are a few modules that I created first, and then Daniel added his own to the core. The differences are typically negligible.
Off the top of my head, the differences between some of the modules in OpenCart core:
Mostly the same. Early on some people were getting stuck on the "Please Wait" ajax load. I used a direct curl post method that doesn't have this issue.
The Core requires password for some reason. This is not a required field for the lookup so I don't use it in mine.
I've updated mine to include the Jan 2011 patch for int'l rates that was not yet updated as of OC v188.8.131.52\
As of 1.5.x, I've merged my changes for USPS into the core to keep things simpler. Anyone using the "USPS Live" mod can just use the one in the core now
It also has the updates for Rate V4 which was put into effect in May 2011. I will no longer offer or update the "USPS Live" extension as there is no longer a need for it.
There are a few features that the core doesn't offer:
- Insurance Option
- Automatic box splitting by configurable max weight
- Option to display List or Discounted Rates
- Additional hidden padding/discounting options for the rates with % support
- Improved error handling
As of 1.5.x, I've stopped supporting "UPSXML" and instead have changed my focus to "extend" the UPS extension in the core of opencart. I've created a separate mod called "UPS Deluxe" to add additional features to the existing UPS extension in the core. I will no longer be supporting the "UPSXML" extension as it is no longer needed.
That is about it. There are also some other 3rd party modules that people have duplicated as well. I don't use them so I can't compare
All modules on my site are authored solely by me. I have always kept my modules working, backwards compatible, and up to date with current versions.