Hello,
support@itch.io is the correct address, sometimes our backlog gets a little high though so it can take us a bit to respond.
The final payout amount is calculated from the sum of all your purchases - the itch.io cut - the payment provider fee. It's likely that difference you're unsure about is the fee added by PayPal or Stripe. If you click into the details page of the payout there's a breakdown of where the funds are going.
Thanks