On the HamRadioHelpGroup mailing list, someone asked, “How do I ask someone for a QSL card without sounding like I am begging for it.”
Mark, K5LXP, replied, “Send them one first. If you really want one back, send an SASE. If you really want one, include a few green stamps (dollars).
“What I do is send the QSL, a pre-addressed envelope and a stamp. It’s the op’s choice to use my stamp or one of their own. At least half the time they send me my stamp back. I always do. If someone’s willing to invest ~50 cents to get my QSL, I don’t have a problem returning the favor in kind.”
I hope you think this is as good an idea as I do.