I suspect your buyer was using the iphone or android app on their phone.

I have both flat rate and calculated postage, and have had a couple of buyers tell me the same thing this week. Postage shows up fine on my end, but they tell me the listing appears to have free post when they view the item, and then when they proceed to payment the actual postage shows up.

Perhaps ask the buyer if they purchased via their phone to confirm this is the issue and then contact ebay to let them know. It is unfortunate that this reflects badly on sellers as some buyers may think we are modifying the postage charge when it is in fact a bug with the ebay phone app.