Before anything, please take a read: www.cregox.com/your-home
I can see no link to screenshot or file attached, but I imagine you might want something like this:
www.cregox.com/shop/ninety-one -> press "add to cart" and try to checkout. You'll see a pop up.
Ignore the rest, this is an old test page. I can make a popup just like that and add a "lightbox" layout to make all the website dim out. Additionally, I could add a cookie to prevent it from appearing multiple times if you enter the website again on the same browser for 1 hour, for instance.
If you want to move forward, this should cost me less than U$175.
That being said, I like much better the way your site is right now and I think there is no popup needed there.
Finally, if you're willing to join, I'm starting with an experimental new ticketing system here. This (and any further tickets) are public discussions!