The problem : when Photobox asks to its users if they want to change the size of their products, the company basically asks "Do you prefer a 20 x 30cm canvas at 25 euro or a 35 x 45cm canvas at 43 euro ?".
This is really bad.
We ask users to make a choice based on a completely meaningless size (who really knows the difference between 20 x 30cm and 35 x 45cm ?) and on an incredibly meaningful price (I bet everybody knows the difference between 25 and 43 euros).
You don't show the value of the size to the user, so you can not expect that he'll make the sacrifice of euros for an unclear benefit.
There is an important difference between configuration and upsell : one comes from a user's need, the other one from a business' need.
The user doesn't need to upsell.
Let's be clear, I don't mean that upsell is "a bad thing" just because it's a marketing push. Options available on the upsell page are sometimes very useful, and they need a dedicated place and time to clearly communicate their values to users who could really find some benefits in them.
But you can't treat them as configuration options. The goal, and therefore the approach, are not the same.
When we worked on the upsell process, we tried to show clearly the benefits of each feature by using visual cues, by limiting the number of options available (so you have more attention to explain each option), and by trying to be fun.
We quickly decided that it's important to show products in a real life situation.
Firstly, it helps to understand the different sizes (comparison between options).
Secondly, it helps to assess the size of a specific product.
And finally, it helps to make the product more tangible (before this step, the user had to deal with very abstract concepts in order to create his product).
We decided to use the same approach on our other upsell pages. On the books' creation tool for instance, we tried to show strong visual of the pages quality to help the user to understand differences between all the finishing available.
Don't hesitate to have a look at other case studies.