> you have to send messages to a central server every time you make a transaction.

This is not necessary. As long as the ecash wallet has enough coins of different denominations, the sender does not need to send a message to the mint server.

For example, Keychat’s ecash wallet will exchange a large number of 1-sat coins with the mint server at once, and these coins are then used as stamps. The sender doesn’t need to continuously send messages to the mint server.