I thought the camping card was a bit of a con, in that it only covered maybe a third of the campsites available which you may only discover after you've bought it.
Going there in Sept meant many campsites had already closed, so finding one close by that was open mostly meant the card was useless. If you don't mind having your route altered by aiming for the campsites in the scheme, and you will be there in August, then maybe a good idea.
On arriving we stopped in Seyðisfjörður for the night, next day bought the card in Egilsstadir tourist info to find that lasts nights stay should have been discounted, and they could/should have sold us the card if we'd known
At the closed campsites we stayed on often had someone come and say hello, and no-one minded us being there. Water gets turned off and plumbing removed, so its just a field
This road map shows a lot of campsite locations, shows dirt/tarmac, and isn't a bad size for a tankbag Ferdakort 1:200000 ISBN 978-9979-67-209-8