As its a completed transaction its in the BST Archive. I suspect (quite sensibly) that the only people with posting rights in the archives are mods or admins.
That is correct, once the Post title has been updated as ### COMPLETE ###
Then it will be put in the B/S/T Archive and you can't edit or add new posts, if you want to add a little thank you comment just do it before you update it as complete.
Bechet, i'm not pulling the Post out of the Archive just for one comment so i have put one in the first Post on your behalf.