I have used Skydemon, which is good but has a limit to the number of free flight plans... But you can also get an account directly with Eurofpl which gives you another set of free filings.
Then with both you have the option to top up with pay as you go credits, which are quite cheap but I have never had to pay... yet
Sadly I think we are moving into a culture where larger companies have removed pretty much as much cost as they can and now they need to increase revenue streams to keep shareholders happy. I wonder how long the free AFPEx will last