Single entry tourist visa for 30 days - is invitation required?

Hello,
I am a Polish citizen, traveling in Vietnam on a tourist visa.
I have just come back from the Chinese Consulate in HCMC. I wanted to apply for a 30 day single entry tourist visa to China. I was told that apart from the completed application form (got that) and flight tickets out of China (got that too: as for entry, I will be comming by land), I also need to have a written invitation or a confirmation of joining a tour. I have been to China before (I applied for visas in my own country though) and never needed such documents. The lady in the consulate told us that this is due to some new regulations.
I do not have any of the last two documents and I am not sure how to deal with it. I do not intend to join a tour nor do I know anybody in China. My questions are as follows:

1) Are these documents really necessary? May this be some kind of local practice of the HCMC consulate or is it a common law now?
2) Is there any way to avoid this?
3) If I were to get an invitation, would it have to be issued by some institution or could it be a regular person? If the latter, does it need to be a Chinise citizen or can I get an invitation from someone on a business/tourist visa?
4) Do you know any agency that may help us with this issue?

Thank you in advance for your help.
Jan