All cards are blank inside but here are some suggestions to help you get started:
"It stands for Happy Birthday, in case you couldn't figure it out."
I'd recommend writing your whole message as acronyms. TWLI, IP. They will love it, I promise.

This card is printed on handmade paper and printed by hand, so naturally, it all has variations in color, texture and print. Perfectly imperfect just like you like to tell yourself you are.