After receiving my first batch, it appears that 6 months is the "best fyrir" ("best until") date. It didn't come in any special refrigerated packaging, but I agree with Ecnerwal that it should be kept someplace cool to help extend the shelf life.
Depending on your climates humidity, if its too humid salt works like a big silica pack. It absorbs moisture, and you might get liquid salt runnoff. If you live in Denver Colorado, you could probably store it on your porch.
General suggestions are to wrap salt blocks in plastic wrapping.
Therefore if you double up that with an airtight container you can ...
In order to preserve your custard applies I would suggest canning them since you have already frezing it. It does lose it's freshness to an extent, however, the shelf life would last longer.
Canning is also a good method as is can use vacuum sealed packaging which restricts air into the packaging which is what causes micro-organism growth an multiplication.