Most of the Retrocards sets I sell have 15 cards, and the total for shipping in a padded envelope is under $2.00. You can find padded envelopes for .50/ea if you buy a small quantity, then other things like card sleeves and team bags, tape, ink, ect, it should keep everything under $3 - and that includes delivery conformation. I don't think $3 is outrageous, it seems to be the norm, but a wise shopper will pass up your single card for another with cheaper shipping costs...if it's a common. What I think people hate the most is when the shipping costs more than the card they are buying.
I'm more likely to look at what else a seller may have for sale when they offer free shipping, or shipping is $2 and under. That's just me though.