Good for you Dianne.  Hold that line.  I've had this happen to me many, many times & really, the only answer is to refuse all further orders until full payment is received.      What always gets me going is the veiled threat that if I don't continue to accept & fulfill orders regardless of what's owed, then I'll be off their list - I just can't stop laughing at the idea that I won't be allowed to do anymore free work.   

The other problem with this scenario has always been what happens after I get paid.  Then it's a matter of deciding whether or not I want to go through this process all over again with the client - the endless emails & their endless stalling.  It's hard to decide how much annoyance I want to put up with, particularly if they have sent me a great deal of work. 

Good luck & I hope you get paid.