The shopping cart can also be run in single order mode, which replicates the behavior of the older single item ordering pages in SOLO. This mode is enabled by passing in Single=true in the querystring of the ordering link. The distinctions between this mode and the regular cart mode are as follows:
Support for product level custom html.
No continue shopping button or upsells are displayed on the cart page.
Author and product level custom mailings are support (standard cart only supports the cart order complete mailing).
Only 1 item can be added to the cart. If another item is added, the original item will be removed.