What do you accept as certification of place of business? Is a business card indicating an address sufficient, or does it have to be a signed document?

A business card is not sufficient. The certification of place of business should take the form of a signed letter from your employer stating that you are employed by that company, as well as the capacity in which you are employed and the address at which you carry out your duties. The letter should be signed by either a professional representative, a member of the HR department of the company, or a person entitled by law to sign on behalf of the company (chairman, managing director, etc.).  

For self-employed representatives, it is sufficient to provide proof of tax/business registration.