Avril Lavigne Concert Lawsuit -- I Did Your Stupid Concert ... Now Fork Over My $375K

Avril Lavigne signed on the dotted line for a huge gig that was supposed to pay her a whopping $500,000 -- but nearly a year after her performance she's still waiting for most of the cash ... so she's suing to get it.

Avril filed the lawsuit against 2VLive -- a live concert streaming company -- claiming it agreed to pay her the half million bucks in exchange for promotional appearances, and a concert in New York City on September 25.

The company agreed to pay Avril in 3 installments -- according to the docs filed in L.A. and obtained by TMZ -- $125K, $300K, and finally $75K. Problem is ... Avril says she only got the first payment.

Additionally, Avril says she got into bed with an affiliate of 2VLive that produces video greeting cards. In the docs, she says she was supposed to get 15% from that deal, but again ... she's still waiting for a payday.