You have advantages if the store you choose is a Pocketbook partner (
https://pocketbook.ch/de-ch...), as the books you buy are then automatically uploaded to the Pocketbook cloud and from there end up on your e-book reader. However, you can also buy books elsewhere as long as they conform to the .epub standard (which I believe only excludes e-books from Amazon). I regularly buy e-books from shops other than my connected shop and then load them onto the e-book reader, no problem.
Since ExLibiris also seems to be a PocketBook partner, it takes a short and simple contact with PocketBook support after the purchase at Digitec so that they can customise the connected shop on the device (see e.g.
https://papierlos-lesen.de/faq...).