Hundreds have processed and I've only seen two that failed to add shipping. I agree replication is the key but so far no luck. I see it as one of two things. Either the user is somehow manipulating the purchase, either intentionally or unintentionally in a way I haven't thought about with this result OR I have a flaw in the code somewhere. I was suspecting the weight as the issue but it must be something else in combination because I can't replicate it with weight alone. I can't even replicate it by repeating the exact same purchases as the original two failures. Very strange. Driving me crazy.