Heimdall's StdTx is not using Fee for each transaction. We have very limited types of the transactions and as end-users won't be deploying any kind of contracts on Heimdall, it is using fixed fee model for the transaction.

Check here:

// StdTx is a standard way to wrap a Msg with Fee and Signatures.
type StdTx struct {
Msg sdk.Msg `json:"msg" yaml:"msg"`
Signature StdSignature `json:"signature" yaml:"signature"`
Memo string `json:"memo" yaml:"memo"`