You can spawn things you can store in your inventory (important, they must be placeable in the mailbox) in sandbox, save the world, change the world type to story manually by going to the save directory and then change b_(filename) to s_ (filename) on the save file name (that prefix is all that dictates the world type), load the now story mode save, put the desired items in the mailbox, save, go to your normal save, take your cheated items out. DO NOT tick the spoiler items on in the tags of the sandbox menu if you want to maintain some semblance of discovery and mystery in the game, and I would honestly recommend just typing things into search without looking until you're done typing because there is still lots of kinda spoilery things in the menu. While cheating, I honestly think this is practically a mandatory thing to know how to do since its so easy to permanently lose important/unique items.
NEVER, EVER CHANGE YOUR MAIN SAVE TO SANDBOX MODE, IT WILL PERMANENTLY DELETE MANY ACHIEVEMENT AND "FUNNI" ITEMS LIKE MAXWELL, THE ARGEMIA PLUSHIES, AND MORE.
Also, while this can be used to make infinite/solar saves story mode and vice versa, back up the save before doing it and expect things to be broken and/or many events to be missed/deleted