You can recieve tokens while you offline and they are hold at the mint for you. Imagine it like an IoU of a free bank. If you don't trust that bank they can send it over LN to a mint you trust and it gets remintet or directly into your lightning wallet if you have channels.
If you have no channels and cashu all over the place it also doesn't matter because it aggregates to one number in your account. When sending tokens at different mints are sent together to the reciever.
