The best practices for a shopping cart would be the same as any other web page. Each cart software offers varying abilities to natively conform to SEO best practices. Many popular carts offer extensions to bridge the gap between the software design and the changes necessary for improved SEO performance.
If I were in your situation I would request control over the URL, page title, header, text, alt text, etc. All the normal SEO factors. If the web developer is not willing to provide you access, then ask the developer to make the changes. Either the developer will likely not be happy with making so many changes and then grant you permission, or the site owner wont like paying a developer to make text changes and he will request for you to be given access. Either way, the result is the needed changes are made.
My best suggestion would be to inquire on the X-Cart software forums as to what SEO extensions and customizations are available.