your wallet (HD wallet) is a private key tree generating from a master node.
your wallet (master node) can generate many nodes each node can have many of children (in other wods it can generate too many private keys or addresses).
So all the addresses are back to one master node (your wallet) which appear in your balance.
Address reuse It is an unintended practice, abusing the privacy and security of the participants of the transactions as well as future holders of their value. [1]
to learn more about transaction fees tutorial: Calculate Transaction fee [DannyHamilton]