Thanks for sending us your link, I was able to test your buttons in several browsers and they are working fine even with ad blocking and pop-up blockers..
I'd guess the people who had issues were using Internet Explorer. We've been working on a bug between IE and our new cart but that hasn't been completely resolved yet.
IE's overall usage has gone down from being the top browser a few years ago to being used by 3 to 5 percent of the web, so most people won't run into any issues. But anyone who is still using IE today probably doesn't have any other options to switch to.
For the short term you can switch back to our original cart overlay from here, that one still works with the older IE browsers:
We'll be able to follow up with you when the IE bug has been fixed if you want to return to the newer cart afterward.