Hello,
I'm wondering if it's possible to run the 2.6 software with the 2.5 USPS plug-in
Yes I think that temporarily using the 2.5 version will work on an Hikashop 2.6.
I can create separate instances of the plug-in, and that works okay for domestic options. BUT, as soon as I put in the international option, I get the error message about not being able to get the shipping rates. I also have uncovered a new error when playing around with the international shipping:
Ok, so I'll try to reproduce on my end and fix is for the next Hikashop release.