Hi,
The popups work fine by default on our end and no one else reported the issue. It could be linked with your template.
Try switching to protostar temporarily to see if you still have the problem.
Also, if you could provide the URL of the shop and a test account with an order for which I could print the invoice, or with a wishlist I could share so that we could check on the situation directly, we will be able to better understand the situation.
It might be a conflict with another extension.
