really really like the function of Scarlett's cuss-box and it's gradual role reveal in the story.
at first it really seems to be Stroud's way of turning the whole "children's books can have characters dying in the most gruesome ways possible, but no one is allowed to cuss" into an exaggerated joke. but it gains depth at the reveal of Scarlett's habit of gambling, of her not keeping the money she steals, at the fact that she favours cuss-box's weight on her to its absence. Scarlett has to physically remind herself to do better, to have something to weight her down, to remind her of a burden she must carry.
She’d dropped them by the river gate, along with her rucksack and her cuss-box and her prayer mat, and was temporarily set free. She felt almost weightless, existing solely in the present, and while that present lasted, she knew only a fierce joy. (TOSaB, ch. 14)
"weightless, existing solely in the present" it's a punishment that Scarlett forces onto herself. she could forgive herself, could move on, but feels as if she didn't deserve it, not when it lingers in her peripheral vision still.
Scarlett's cuss-box also seems to have something to do with karma or the way Scarlett perceives it. when she does something wrong, she loses that money, but it means that someone else, someone who's done something good, someone more deserving of that money than she is, should get it. (slave girl in Lechlade, people that took them in for a night before reaching the lagoon).
and that's why i love the fact that Scarlett loses her cuss-box once she gets washed up on Free Isles. because it has two meanings: she upheld her end of the deal, she got Albert to the Great Ruins, she did something good for others, it should unset the weight on her. and it also means, that she shouldn't feel guilt for saving Albert back on the raft and freeing his powers. Scarlett in the moment thinks that she caused everyone's death—Albert's, Ettie's, Joe's—but that's not what happened, her actions saved everyone, she shouldn't be weighted down by that guilt. that's why her cuss-box was lost, that's why she's free on the Free Isles