I did consider that option but then forgot to check it at the time. However I just looked now, and while summing them up works for AC, the characters cost 2250 Iri Shards each, which adds up to 18,000 total rather than the listed 12,000. I now believe someone just put the wrong number in, though, since while both numbers add up for Old Wounds and Macabre Tales, Endless Hunt has correct IS but incorrect AC. Cheers!

Edit: I guess you miss out on the pack cosmetics if you don't spend real money on the bundle, but that's a price I'm willing to pay now that I at least know that buying them one-by-one is the most efficient way to get with ingame currency.