For a start, because inevitably people think of this..., I did get those zip pulls for free but I am not making anything out of this apart from entertaining myself.
I did a finger pull ring with one of them, no problem.
So I increased the pressure eventually trying it with a 10kg (22lb) load.
It held for about 10 seconds than it snapped
if you put anywhere near that much pressure on it you have far too much pressure on the zip.
Of course you could always have a faulty one but at 35c each (lots of 50..)a spare or two would not be a bad idea.
BTW, in a tipi (or similar) were the seams (or zips) act a bit like a built in guyline, you will have more pressure on the zip, so maybe you should avoid this type of zip pull if you have really heavy duty zips.