Generally any action that requires blockchain computational resources to be performed is subject to a fee - such is the design of the blockchain. This includes wallet initialization (a small fee will be deducted from the first coin transfer you receive), coin transfers (the sender pays the fee), system messages from DeBots to smart contracts on the blockchain.

Chat messages in Surf are not subject to a fee.

More information about the Free TON fee calculation can be found hereThe original source of fees and calculations is a Telegram' team description about TON blockchain.