I've been confronted with the "buying meat is ok because your contribution won't make any difference" argument about half a dozen times throughout my life.
But it's a non-starter! Purchasing meat is very unlikely to make a difference, but it has the potential to do an enormous difference! Maybe your purchase will push the store's numbers over a threshold, which will lead the company in charge of producing the meat to increase production.
So if X is the random variable measuring impact, then E(X) is the product of p (the probability that this happens) and let's call it z (the impact if it happens). And E(X) = p*z will come out as almost exactly the same impact that you would have if someone were killing animals for you personally. To see this, you need only imagine that a million people became vegetarians tomorrow. This would decrease meat production by about the amount that they consume, therefore the impact of each one is 1/1000000 * {amount that they consume} ≈ {amount that one person consumes}.
It's a few discrete jumps as supposed to a continuous curve, but it all nets out to a normalcy behind a veil of ignorance. Ditto voting (if you live in a swing state). So many people don't get this!
In both cases, you also can't tell whether your contribution made a difference. In the case of elections, the discontinuous jump is not actually one person winning with one more vote, it's whatever makes the election close enough for a recount, which then leads your candidate to win after all. You don't know where the threshold is there, but there must be one. if 10 vote difference means recount and 10 million means no recount, then some number n between 10 and 10 000 000 is the threshold.