What are the costs? - One time purchase fee then nothing!
What does error *** mean? - Please make a support ticket when you get errors. (see below)
What makes this better then ... Donation system? - its the only one fully ingame beside the fact that others limit if you do not pay them monthly this one is one time all features!
Is it secure? - Yes it certainly is it works with a encryption and key system basically a pincode thats encrypted and compaired to the server his own key beside the fact that it never stores any personal information only the ingame nickname and products purhcased
Is it customizeable? - yes you can translate it to the full change colors and icons used in gui's
Does it work on versions lower then 1.13? - Yes but i will not give support for it