I use virtual credit cards to perform online purchases with my debit card, and I usually create a new one for every purchase (with a charge limit equal to the purchase's value).
I was trying to add a new one today to buy an app on the Mac App Store, and I got a message saying that it couldn't be added, as there were already too many credit cards associated with my account.
I did a live-chat with support, and they told me that there is a 5-card limit every 30 days, so apparently I can't perform any purchases for 30 days.
Is there any way around this? I don't have iTunes Gift Cards readily available in my country.