Also Bought CBB

This app's automatic feature is what you want from the app. It intelligently picks out related products based on an algorithm that allows to appropriately display items of interest to your buyer. Note that you will have had to have a moderate sales history for the past year or two. But not to fret, manual, global, and random recommendations are available too.

I have used Cross Sell's app for a while and find their pricing too much for what they offer. This better app is offered at half the price and if understood, can do quite a lot to recommend the right products to your client.