Bad service

I have been struggling since January to get a credit on an over charged invoice. We ordered panels in December and paid for it, but their stock ETA dates changed to January. In January when they received stock they still invoice my the higher price from December while selling it at a lower price. 30 Panels makes a huge difference, so I contacted them and they said telephonically that they will credit the difference. I am still waiting. They just keep forwarding my e-mails to each other, CC me in it but no one is coming back to me or do anything about it. I am now going to use other suppliers when possible because I can't stand their bad service anymore!