Marketpress order quantity and Paypal payment quantity don't match

See attached screengrabs - my client at got their largest order to date yesterday, but one major issue came up.

Customer on the site looks like they bought 3 sets of letterpress cards, but were only charged for 2 on paypal. This is the first time I've ever seen this happen, any ideas as to what caused that? Client wants to ensure it doesn't happen again.