Why was my customer charged to receive their package?

• usps postage machine
• discount office chairs
• receive online payments

I shipped an item (that I've shipped dozens of times before) from the post office in a small envelope and it costs about 50 cents. One of my customers recently informed me that she was asked to pay $1.80 to receive the item when it was delivered to her business. When I asked her what their reason was for charging her she said that she didn't know but assumed it was due to insufficient postage. Because I ship this item from the post office on a regular basis I know the shipping was accurate so why was she charged? Also I ship much heavier and larger items and nothing costs that much to send. She didn't ask for $ but I want to give her the reason, or if it was my mistake a refund. Thanks

Why was my customer charged to receive their package?