Fashion Nova review: My shipping was changed

I placed an order yesterday that was over $100 so I could get free overnight shipping. I changed the shipping from standard to free overnight shipping. I made sure I did that then I proceed to pay with zip pay. After I checked out Zip pay I returned to the site and placed the order and you guys automatically change my order from the free overnight shipping to standard shipping. I am livid because I selected Free overnight shipping in the system. Still changed it to standard shipping. This is unfair and I want my order shipped out as soon as possible because that was the only reason I spent $100 was to get the free overnight shipping.