Miss Blueberry
Cuberoot
It's popularity peaked before my time, but when Soeur Sourir (The Singing Nun) topped the charts with Dominique-nique-nique, it must have enjoyed significant secular popularity, perhaps in part because they didn't know French well enough to really understand the lyrics.
Occasionally, maybe once per paragraph, misspell a word intensionally. Your family, knowing how carefully you used to profread your own writing, will notice this as abnormal behavior. Either you captors have already damaged your menial health or you are trying to conceal a message. Gards reading you letters before posting them may be more used to bad writing among their detaines and not suspect anything deeper. Your family might reply using the same code, both acknowledging receipt of your coded message, and perhaps including a key for a more secure one.
They should round up.
Somebody's probably already written a mobile app that calculates how to split up a purchase so that the price rounds down as often as possible, and you'd willingly pay 4.9 cents to avoid being in line behind someone using it. There's only one solution that incentivizes doing what's most courteous for the other shoppers by paying for the whole cart as a single transaction. Maybe they'll donate the excess to their pet charitable foundation, but they need to round up.
$123
spoiler