@Iain Howe  I can understand where you're coming from as a merchant; I reviewed the thread and found the investigation on the matter which has since been resolved. 

Many performance issues can be alleviated by basic browser maintenance.  

Heavy browsing can lead to the build up of cache on the back-end which can sometimes prevent some programs from opening or functioning correctly.  Suggesting to clear cache or use incognito (private) browsing will usually allow for a smother experience. 

Have you tried sending the invoice to yourself to see if the page opens for you? If not, test it yourself before calling your customer with the suggestion to troubleshoot their browser.

Let me know if that helps.