[Starkel]

With dry ice you don't need a tub or anything to put it on so it doesn't dissolve on the way there or?
Because im thinking if i do it in a envelope it will dissolve and just leave water marks all over the envelope

Dry ice doesn't dissolve it sublimates, once it sublimates entirely there will be no residue.

Bear in mind it is incredibly cold so think about how that may impact putting it in an envelope.

Where can i buy dry ice?

It is a dangerous product, be careful with its handling.

And make sure to pack it well. It can turn against you.