I have a jar of preserved lemons in my fridge, but I made them from scratch myself because they are harder to find than they should be. Apparently this is because everyone who uses them regularly makes them at home and demand is low, which I think is spurious reasoning.

And here is where I confess that my tamarind paste jars are usually years past their so-called expiry dates -- it's not something that goes bad.

But, yes, buying something you don't normally use when you only need a tidbit of it is absolutely a waste. A cookbook author I respect calls these recipes "a quarter of a quail egg" recipes and tries not to write those sorts. It's why I rarely make Ottolenghi recipes -- they all have fifteen ingredients, five of which you'll never use again.