_snailmail_print can be called on multiple letters, and this can lead to a timeout from IAP Service. However, letters are sent, credits are consumed, but letters aren't marked as sent. To avoid that, we force the sending to IAP service to be done letter by letter. closes odoo/odoo#68107 X-original-commit: 3d8ad1e68583fa1b354ff36f902ccb0405270bb2 Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>