Unsigned contract enforceable because of conduct?

I made an 18 month independant contractor agreement with a Philadelphia company that commenced on May 1 2014. I was to provide general business consulting and Invoice and be paid upon receipt on a monthly basis. A clause in the agreement allowed for the charge of 1.5%/mo for late payments. And there is a no waiver clause. Although he promised many times (even in front of others)over the course of the 18 months during which I provided and was paid for said services, albeit late many times, the owner never signed the agreement. I repeatedly provided other copies upon request and even at times without any request, upon expiration and then non- renewal of the agreement, I submitted a final bill including the interest. The owner snidely responded via email that I would need to show him a signed copy of the agreement stating interest charge rights. Is this agreement, although unsigned, enforceable under the conduct of parties concept? Thanks-