Author here. Are there any other important payment solutions we should be looking out for (available in Europe, Belgium) that I forgot to mention in the blog?
As I just posted in the comments on your blog, Google Wallet is a serious consideration if your app is available in the Chrome Web Store (or on the Play Store). They charge relatively little and support European accounts and subscriptions.
There's also the Google Apps Marketplace to consider if your app targets the enterprise, which also hooks into the same payments APIs.
FastSpring is worth a look. My company (also European) has been using them successfully for subscription billing for over a year. I really like their system.
Klarna is a payment startup in Sweden focused on invoices and installments. Currently working in se,dk,no,fi,nl,de,at. Could be worth keeping an eye on.
There's also the Google Apps Marketplace to consider if your app targets the enterprise, which also hooks into the same payments APIs.