National Trade Services Will not pay for services completed Smithfield RI

I was contacted by a representative of this complany that I was familiar with from another company.  She convinced me to complete two work orders for National Trade Services.  I completed the work orders to everyone's satisfaction, submitted all paperwork and photos as requested.  Payments never came.  Excuse after excuse after excuse.  And then they just started to ignore me.  I explained to them via email that if the invoice went to 100 days I was going directly to their client for payment.  On the 100th day I emailed and let them know I was on the way to the jobsite.  I got a call ten minutes later and they gave me my payment instantly.  Now I'm doing battle for my second invoice.  Truly, giant waste of time so that they can steal $325 from me.  It's not worth my time to drive to that jobsite as it's far away for me so this time I'm making my displeasure known online and I'm filing a mechanic's lien on the jobsite.