im not a posek but i can tell you from a rav i asked-being that im in a business that deals on Shabbos-that as long as there is work towards that amazon sale or ecommerce needed to be done before Shabbos &/or after shabbos (posting the sale is done before shabbos, shipping & billing is done after shabbos upon completion of sale etc…) then it is fine.
to add a note if you will say the 3 steps of 1)posting sale 2)selling it (on shabbos) & 3)billing & shipping is 3 separate things & the buying is only done on shabbos & a sale then you are incorrect. IT IS ALL ONE BIG JOB why? cause without posting you got no buyer, without buyer you have no billing or shipping work. thus it is all 1 job for everything involved in each order rather it is done before, after or on shabbos.
still double check for yourself with your LOR but everything i wrote is from a LOR i asked regarding my business on shabbos