One of the journals (Q1) published my article after the peer review was complete. Now they are asking for APC, I requested for APC waiver, but they refused because I received funding for data collection and transportation which was a very small amount, and I clearly described the situation to them with a letter from the department head. I tried to collect APC from different sources but failed. Today they sent an invoice showing the due amount of 1600 USD. What to do now?