login
1.You log in with your account or via or with your account
2.Your documents have been sent out by the express company or via express company. PS: Is it called express company?

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Also learn the phrase "by courier".  Use "by courier" and not "by the express company".

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You log in with your account.
You can send documents by or via express Mail.
By or via is fine.

Answer

1. You log in with your account
2. 'sent by' and 'sent via' are both correct. "Send via" is a more formal and technical phrase that implies sending something through a particular channel or medium, such as courier or email.
3. Yes. it is called an express company.