As good as it sounds in theory, the FCC has ruled against fax spam.  So, if you did do this to them, they could actually file a complaint against you.

Instead, hire a good collection agency.  There are people out there who get their thrills at bullying businesses who do not pay their bills.

And, to be honest, it is worth the 20-30% collection charge just to know they are getting hounded by the best.

All legal too.

Why not just call them, leave a message that the account is going into collections at 25% annual interest rate (retroactively), fax them the same, send them an invoice with the new amount including the 25% interest and then tell them:

"If your company is having financial difficulties, I will gladly work with you on a monthly payment schedule if contact by ________ date.  If I do not hear from you by _____________date, your account will be turned over to a professional collections agency and I will no longer be able to offer you terms on your account."

It has worked for me!

Best of luck,

Victoria