Hello,
Thanks for providing a detailed description of the issue. Your funds are safe in your wallet, it’s just that they were not credited to that WordPress user in the DB.
1. You have verified that the IPN endpoint is accessible from your browser, but it also has to be accessible from CoinPayments. If you are filtering IPs by geolocation on your site then that could be one problem.
2. A security plugin could be blocking the IPN message. If this is a test site, you could try to disable your security plugins and try again. If the message works this time, this means that your security plugins need configuring.
3. You say that the IPN message was not sent. If you open up the details of the message in your CoinPayments account there might be more information there. Go again to the IPN history, find the message with status = 100 and click on the date/time on the left hand side. This will open the details of that message and there might be an error message there. If there is, please share it with me so we can figure out what happened.
You do not need to test with real crypto, you can test the mechanism with LTCT to make sure that everything works as expected. You can initiate Litecoin testnet deposits from here: http://testnet.litecointools.com/ The IPN mechanism is exactly the same for all coins.
Please keep me updated.
kind regards
Alex