I buy my books from all over the place. I use adobe Digital Edition to validate the digital content and then I load with tolino.webreader. The synchronisation on the e-reader is done by wifi. Unfortunately only Orell Füssli is available directly from the e-reader...

For direct use from the e-reader, only Swiss-German shops are available unfortunately. On the other hand, you can upload your ebooks bought elsewhere on the tolino application (smartphone or browser unless I am mistaken), so on your e-reader

Use the Calibre application and you can transfer anything you want.