X-Cart just noticed it yet. And of course, i guess they live in fear

everyone would get this mod and doesn't need to buy a 2nd license *hehe*
Regarding Add Ons:
Of course the Add ons will only work with the domain you licensed it for. (If the mod has been encrypted). But for example:
Before i had X-Cart to finish my project, i contacted WebsiteCM and the answer i got, if you buy a mod, just add ALL your domains into the primary domain field and add it to your cart.
The same answer i got from Altered Cart.
So i would suggest ask them if its possible to update the mods with your new domains.
To have an addon working for your multiple domains, just upload the core files once, and copy the tpl. files of your addon to each skin folder.
