Yes, it's true. I once did a card (two actually) for Hallmark. I was hired to do two birthday cards for their Shoebox line sometime in the mid-1990s. I had been a (hard)working illustrator for a while at that point, taking any and all jobs I could get. Hallmark dangled the prospect of keeping me busy with regular gigs, so I happily (and somewhat cynically, I admit) gave it a shot. After all, isn't that what young illustrators always hear from their annoying relatives: "Have you tried sending your work to Hallmark Cards?" So, one day I did just that and, sure enough, they called.
Unfortunately, my work for them apparently didn't meet with the approval of one (or more) of the higher-ups and those two cards were all I ever did. They had sent me their illustrators' guidelines -- a brochure that included descriptions and showed examples of how to successfully draw in a "Shoebox style," but I had to ignore it, only because it would have been so unnatural for me to draw in the way they described (in short: very, very cute) - I simply couldn't do it.
I'm including my first sketch for this card. You can see they approved everything but the central girl's face. Then, after a couple more sketches, I got so confused about what they wanted her to look like that by the time I did the final card -- you can see -- her face looks almost abstract. Oh well... One of the good things was that they let me do my own lettering and even draw that toy bear on the back.
I was a bit disappointed that I didn't make the grade for Hallmark -- I wanted work. But I wasn't surprised. I'd had a lot of worse gigs and (luckily) a lot of better ones, too. Anyway, a couple of years later I was so busy I had to start turning work down -- then, eventually, I was able to start concentrating on writing & drawing comics instead.