I visited the Van Gogh Museum a few years ago and at that time I did not have the opportunity to buy a large print. So, I was happy to have the option to do so online on your museum shop. I liked that I could click on the different sizes to get a visual of the print sizes I was selecting.
Improvements or suggestions
However, overall, I found your shop website clunky and difficult to use. The shopping affordances of the website compare poorly to other similar online shops. Whenever I was in the shopping cart and changed an item, I had to go back to the home page of the shop and start over. Several aspects of the shop's website were a bit unimaginative.
I also suggest that you make this feedback form more specific based on different categories that customers look for in an online shop.