Buyer was not charged postage

I'm really confused as to why this has happened but I just sold an item that was listed with an extra $15 postage cost however the buyer was not charged postage and was processed with free postage. Even when I go to sell similar, it says $15 postage cost. I have reached out to the buyer but am waiting for a response. Has this ever happened to anyone before? Any advice/help would be much appreciated. 

what is the item number?

375203246542

 

Did you offer free pickup? Did the buyer select it?

I did offer free pickup but the buyer selected postage

I had a $15 postage charge listed on the item when I checked earlier today. The buyer just responded saying that they has eBay Plus that gets them free postage on the item, which is ridiculous, considering the the total after fees is lower than the postage cost I charged initially.

all your items for sale have free postage

Yes that's right, but not with this one. I had local pickup selected for this one as it was a bigger item plus the additional postage charge. 

Thank you all for the responses. Did a bit more digging into eBay Plus and have another question. They offer free express postage but mine was listed was standard. Would I need to ship through Express post?