My experience, as a reader, with a number of online bookshops is that even though they offer e-books, they are extremely limited in variety and mostly old editions. They offer nothing modern and recent. The purchase I made just to try it was very interesting and the books themselves were far from the quality offered by English-speaking shops: there was no content, a colour cover, links to the various chapters of the book and other nice features that are within the capabilities of modern electronic publications.
As for the purchase of paper editions, I can say that it has not evolved from about 10 years ago when I ordered my first book from an online bookshop. The shopping systems themselves have evolved as well as the payment methods. In addition to the already mentioned payment upon delivery, in the majority of the shops you can pay with a card or through any of the popular online payment systems.
In the end, I will mention one popular program that serves as a catalogue of e-books, converter between popular file formats and e-books editor. The software is free and cross-platform and is called Calibrе. Through it, you can easily manage your virtual library and upload books on various mobile devices such as e-readers, tablets, smartphones.
GRReporter opens its own online bookshop that will offer contemporary Greek authors, translated into Bulgarian language.