Is there a function where an add on sale product may be suggested at check out? Such as "Add on a xxx for $5 more?
Yes you can. You might consider the Related Product Upsell Overlay. This field controls whether and how Related Products are upsold to customers in a modal overlay on the first screen of the checkout process (usually the Login screen).
If the Upsell Overlay is in use, Related Products appear in the overlay based on the current contents of the Shopping Cart, subject to the Upsell Maximum Products field. Prices are included, as is either the Add To Cart or Select Options Button (Layout/Buttons), as determined by the Add To Cart Type field.
When the Add To Cart button appears, Related Products may be added to the Shopping Cart in a single click. The Upsell Overlay may be operated in one of 4 modes, which concern the behavior of the overlay after one of its Add To Cart buttons is clicked.
Auto Update: The Upsell Overlay updates based on the new cart contents, potentially displaying products that were not displayed previously.
Static: The Upsell Overlay remains static, and does not change.
Auto Remove: The product added to the Shopping Cart is removed from the Upsell Overlay, while the others, if any, remain.
Auto Close: The Upsell Overlay automatically closes.
You can find this section in your Order Management System/layout/main layout/product detail.
For more help, please contact your account manager.