Git: best advice that most people do anyway: Don’t keep track!
As the old adage goes, worry for your Ruchnuos and other’s gashmius. Who cares if it’s right or wrong? You’re not going to mussar them about it anyway, nor should you. If you don’t keep track you save yourself bad feelings if the card never comes.
And Syag, thanks for your openness. Postage stamps and Thank you cards are more expensive than people realize, and end up in the trash. But it’s good to have Hakaros hatov, so sometimes a thank you phone call will suffice.