Whether it is worth it depends on how close your purchase is to that $50 versus what the shipping cost is.
For instance;
If what you need to buy is $30
and shipping is $10
that's only $40
And buying enough for free shipping would possibly waste $10.
On the other hand:
If what you need to buy is $45
and shipping is $10
that's $55
and you would save $5 by buying $50 worth of items.
It also depends on how many time a year you buy from there.
I buy a lot of hobby stuff online simply because there is no hobby shop within a hundred miles or more of where I live.
And most online hobby sellers have a 'buy this much and get free shipping' thing.
Sometimes it works for me and sometimes not.
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"There are a thousand things that can happen when you go light a rocket engine, and only one of them is good."
Tom Mueller of SpaceX, in Air and Space, Jan. 2011