Don`t worry! It is a common practice.
When custom writers come to our company (handmadewritings), they work on a trial, as well.
I wish you luck!
Maybe it is in Anglo-Saxon countries or Eltzin Russia but it is not the case in the country whose resident I am. People here work for 2-3 days on trial max. This was a non-heard of arrangement. No person born in this country would allow to be humiliated like that.