Father's Day

Talked to my dad twice today. He had received our card, even though I'd unwittingly put a 39-cent stamp on it. I went to the post office last week to buy stamps out of the machine and inadvertently bought 39-cent stamps. Why are they still selling them when they're outdated? I didn't realize I'd bought the wrong stamps until after I posted the card. Does caveat emptor apply to the post office now? (I'll be more astute next time.)