Never heard that saying before. Is that something the author made up or is it something existed somewhere before?

Titanzer said:

Never heard that saying before. Is that something the author made up or is it something existed somewhere before?

It's British* in origin, and it's still used in the Anglo-Saxon Commonwealth (UK/Aus/NZ/Canada/etc.).

(*Okay, first attested use in written form is IIRC Australian, but the Brits use it more often.)

Granted, the original Japanese is just saying, "Thanks, I'll treasure it" though.