What you need to learn from Dunkin Donuts
One need only go as far as their local Dunkin Donuts to learn how to better monetize their own software. Give it a try... Go into D.D. and order just a Coffee - at the P.O.S. you WILL be asked "Would you like a muffin with that, or perhaps a Donut?".
Now Dunkin Donuts does not have a monopoly on "second selling" the customer, far from it. When I was just out of school, one of my first jobs was working in a retail clothing store. Every sale was the same... Customer comes in looking for a pair of pants. They would bring the pants to the cash register, and the first thing I would say is "I have an excellent shirt that matches those pants perfectly, let me show you...".
So what this means to you, is that at the point of your customer's shopping cart experience where they are about to hit the Buy button, show them an additional offer - preferably a product bundle where they can save even more money should they purchase more then one item. Only have one product that you have made for sale...No problem, there are ALWAYS other peoples products that you can offer. If you are a low volume vendor, you can easily use affiliate systems to locate others products that have synergies with yours. Should you be doing higher volumes of sales, consider reaching out directly to other companies, where you should be able to make yourself a better deal then what they are paying their average affiliates. Do NOT be afraid to ask if you can white label their product, this is even better as you get to brand your products, and not someone elses.
The Digital space is no different then brick and mortar sales, second sell them!