It's only a problem with Qatar Airways. I've done it many times with other airlines with no problems, but Qatar Airways makes it into a big hassle. For Qatar Airways it is much easier to pay in cash (use your credit card to get the cash). I was amazed that even though Qatar Airways was paid in full by my credit card issuer far in advance of the flight, they still claimed that they needed to "protect" me from buying a ticket for my family.