Perhaps. You have to consider how your product is used and how the users will sensorially interact with it. If you have a website, for example, you could probably assume similar US and UK usability -- but now consider layout for Semitic languages which are read right-to-left -- or consider ideog...
You can of course use analytics that will give you statistics, but will not answer "Why?". Gathering Customer Feedback is crucial and You have to be preprared for low response rates. For better understanding of Customers' interactions with Your site You can use tracking/heatmap solutions (i.e. Cr...
Sorry to be a bit blunt in my answer, but the client hired your "e-commerce web agency" to do e-commerce conversion optimization, yet your company doesn't know how to do e-commerce conversion optimization? Perhaps have a conversation with someone in your company or talk to your manager or the ...