GP 2013 has a new suggested sales items feature. In GP you setup items to be suggested when an item is purchased. So if someone buys a printer, the SOP transaction will suggest a cable or extra ink to go along with the printer.
In Item Maintenance windows there is a new button, Suggested Items. Click the button and added the suggested items. You can also control where the suggestion will occur, on Quotes, Orders, Invoices or Backorders.
What it won’t do for you is determine what to suggest. However, two years a go I posted code for a shopping basket analysis to show items frequently bought together. You could use that to figure out what to suggest.