It was very easy. But the shipping fee was a bummer as the hat I ordered was only available online and not in stock at the shop.
Improvements or suggestions
It would be great if there could be a "click and collect" option. When ordering online, it would have been a help to order online and pick it up at the museum shop. Then you can save over 10 euros on postage to countries outside of the Netherlands (for my order). If you listed this option on the ticket order form, it would give upcoming museum visitors an incentive to order online prior to their visit--and let's be honest--people order more online and that is better sales for you. It also gives visitors a look to see what is available at the shop.