The shipping you set up is based on each individual item weighing one pound. Your buyer is located in zone 3. For every plate, eBay is adding the weight together and charging buyers the retail rate.
1 lb charges $8.35
2 lb charges $9.10
3 lb charges $10.15
4 lb charges $11.00
5 lb charges $11.75
Your buyer purchased 5 plates and paid for a 5 lb package.
You were charged and paid for the online rate $18.76 of a 14 pound package. If your package exceeded one cubic foot in size then you were charged based on dimensional weight instead of actual weight.
Your listing does not include any measurements so it's impossible to guess if you entered the wrong weight or package dimensions, or if your box was oversized causing the higher rate.
What were the box dimensions and weight of the package you shipped, @jsn321716?
