You can define custom item delivery letter for each companion. This is not necessary, because if no letter is defined for a companion, the default letter will be shown when player tries open the package with items which they are saved in companion bags while adventuring. After this message was shown and player click on the package again, the items pops out.
How to define letter strings
Create a json string record for key prefixed with purrplingcat.npcadventure.bagItemsSentLetter_<NpcName>
in resource Data/mail with Content Patcher.
Example
{
"purrplingcat.npcadventure.bagItemsSentLetter_Abigail": "Hi @,^^You forgot some items in my bag, so I've sent them back to you.^I had fun yesterday, I hope to see you again soon!^^ - Abby",
"purrplingcat.npcadventure.bagItemsSentLetter_Maru": "Hi @,^^Blah blah another bag delivery message - {1}"
}