Is the office in your country or not?
If not, how do you solve this problem? "Is there anyway to control the offshore address while I am not in there so nobody except me can see the documents?"
Please note the following:
(a) First of all, as Sols mentioned, you should work with regulated service providers, such as law firms.
(b) In most of the cases, low prices reflect the lower quality of services.
(c) When entering into an engagement with a law firm/service provider, such engagement should regulate the powers of the nominees, how they will be handling each case, including your mail, and they manner by which they will be taking instructions. Not following this will deem the provider in breach of their contract with you.
The above is not a guarantee, but it provides some comfort.
Happy to PM you to discuss your requirements.