You do need a credit card to pay Amazon. BooksOnBoard is not just some random website. They have been around for years and are highly reputable.
As for tinkering, sometimes it's a good thing to tinker. Some of the fonts S&S have used with Trek eBooks have been less then stellar for an eink display. Their use of Adobe Garamond Pro is not good because it's too light weight. The same issue exists for Kindle eBooks in KF8 as S&S uses the same fonts.