Hello,

you have to inherit the form view (snailmail_account.account_invoice_send_inherit_account_wizard_form) and remove/invisible that options.
because that option comes from snailmail_account.account_invoice_send_inherit_account_wizard_form
view and The snailmail_account module is installed automatically.

Another way is if you  uninstall this module then the "Send By Post" option will remove.