Good luck with your app Stefan. I'm generally very happy with Shopify but was disappointed that I couldn't upsell/cross-sell with handpicked items next to a product. However, there's a Shopify App called Cross-Sell which is easy to use and does exactly that - you can choose which products you want to show next to a product. You can even choose products to show below the Cart 'before you check out have you considered...?'
It's great and has just passed Beta testing. Feel free to try it on my store (click on an item of swimwear). www.airymary.com
