Okay, I’ve checked this beta plugin, and while you may technically be stopping the issue for the seller, it doesn’t help the user at all.
I guess I figured you’d fix it by doing the calculation server-side and using that to add it to cart, or to at least suggest the user enable javascript for the form to work. The warning is in slightly broken English which would also make the user feel like the site is less trustworthy. If you are only going to warn the user and not do the cost calculation server-side, at least change the warning message to something like “Sorry, an error has occurred. Please enable JavaScript or contact site owner.”
Hopefully, you can smooth this out a bit more before launch, I don’t want to scare away users with a message that confuses them more than helps.