Flip's Islands of Providence by Annie Fellows Johnston

The next day at noon, after a hurried lunch at the restaurant, Alec stopped at the post-office on his way back to the factory. He wanted to add a few lines to the birthday letter which he had written Philippa the night before. He wrote them standing at the public desk; then, drawing the old wallet from his pocket, he took out the long-cherished gold coin from its wrapping of tissue-paper and dropped it into the envelope.

"I'm afraid it isn't safe to send it that way," he said to himself, balancing the letter on two fingers. "It is so heavy that any one could guess what's in it, and it might wear through. I did want her to have it in gold, but I suppose it will be more sensible to send a postal order."