I looked at the code in a few of your pages and tested the cart there, and it all looks fine and works fine in Firefox from here. Our office ISP is a consumer-grade cable service unrelated to our datacenter provisions, so any browser tests we perform are "real world" without using any privileged/inside network that has any special access or particularly close routing to our servers. If you could provide the URL of any specific pages that buyers reported a problem with, we'd be glad to examine those in particular.
It seems likely that the problems reported by affected buyers are just a matter of their ISP having trouble making a connection to our servers. When someone is browsing your page, the cart button images, along with our box.js script that manages the overlay-style cart display in your page, are all loaded directly from our server rather than from your site's server. Every time our buttons or cart are displayed, the buyer's browser has to communicate with our server to load the button images and look up that particular buyer's cart contents, your product and account settings, etc.
If there's some sort of network routing glitch anywhere along the path across the Internet between the buyer's ISP and our datacenter, that could impede loading of the button images or the cart display, and this may affect all users of that same ISP, meanwhile connections to the rest of your site would remain unaffected. If the glitch is happening with a major backbone provider, it could affect all ISPs that rely on that backbone to handle any significant amount of their Internet traffic. Typically these sorts of networking glitches are cleared up within a day, but more severe disruptions (e.g., a backhoe accidentally cutting a major fiber-optic cable) can sometimes take longer to remedy.