Since march we experience troubles on the grid with items not arriving in the customers inventory. Untill now i found two scenarios, first where the items dont arrive at all, second where the items arrive but poof on rez.
If the items don't arrive at all, have the customer check his/her mutelist to see if you or one of your belongings are on it. This is causing exactly this behaviour, all seems to go well, the payment is taken, the vendor knows no better then the item delivered, only they never are. The customer seems to be able to contact you, only all your replies will be muted, making it look like you never answer. Have another av with you when you deal with this matter, so you can actually talk to the customer, getting the references out of the mutelist solves the problem.
If the items arrive but poof on first rezz theres little we can do but to trust our customers on their blue eyes and redeliver.