Why won't you take my money!
So it seems some guests are having trouble adding items to their cart or logging in. It seems these problems are caused by your browser blocking our cookies. For whatever reason some browsers (or browser extensions) are mistakenly treating our cart's cookies as if they were third party advertising cookies.
By far the most common culprit is Safari, so let's start there. To get things to work you'll need to set the cookie blocking to never, just like so:
If you aren't using Safari then chances are either you manually set your cookie blocking unusually high, or the problem is a browser extension that thinks it's helping you out by defending you from the forces of online advertising evil, but instead it is preventing you from engaging in the delights of mutually consensual economic intercourse. Check to see how your cookies are set, and disable any potentially offending extensions.