Actually the main coding was done under 1.3.7 and runs fine on it. PTC found that it would not run under 1.3.8 (see previous posts in this thread) and I've updated the code- but it's still set to run under 1.3.7. The only warning I can give (with your heavy modded site) is that I have to make 2 changes to an admin core file (order.php). If this has been changed a lot use a program like WinMerge to insert the differences (it's only two lines)