We've got the design screens ready, but they havent yet been put to html/css. Would you test the screens now or when in html/css? Why? Any good advice for testing ecommerce product pages? Good questions to ask?
Do it now.
"With a paper prototype, you can user test early design ideas at an extremely low cost. Doing so lets you fix usability problems before you waste money implementing something that doesn't work." -- Jakob Nielsen
It's not just about fixing "usability problems," but verifying your design decisions.
The link goes to Nielsen's article on the technique and includes links to a book and a video tutorial you can buy.
(YouTube is full of free demos, though.)
Most of UX designers test their ideas as soon as they develop them, on the piece of paper, a rough drawing. Then using a wireframe program like Axure or Balsamiq to create prototypes, test them. After the designs are done they could be imported into Axure to give a better look and feel for the next testing stage hopefully by this stage most of functionality is worked and by the time it goes to developers they know exactly what needs to be built.
If you have designed most of the pages you could do some paper prototyping: http://alistapart.com/article/paperprototyping
Instead of asking questions, especially leading ones, give the users tasks and observe how they preform, where they struggle and where they make mistakes, or find mistakes in design. Take notes and change the design accordingly.