Indeed, Simon is correct. Just search in the osCommerce options and you should find it yourself.
Now Simon, don't be so shameless on putting spam about your business there
EDIT: Thinking of it, I don't see any harm done to your client in asking for a sign up, after all, it's a way for you:
1.- Keep track of your returning customers
2.- Registration is 1 time only, and they can only login and the process is speeded up, since osCommerce then learns who you are.
3.- Amazon (sorry, if I bring that up again, but to me it's an example on e-commerce that everyone should use as a guideline), have the same process, and after you sign up, you can then use features like 1-Click which lets you go directly to the Checkout process when you buy 1 item. The sign up process isn't eliminated, the shopping method for current customers is just made faster.
HTH,