Can seller add additional shipping charges after item is already paid for?

[canadamom_of_2]

I made a purchase and paid for the $3.60 that the seller wanted for shipping. I just recieved an email this morning that they want to invoice me an additional $3.85 for shipping. Can a seller do that? It's already paid, shouldn't they just have to take the loss?

Item number was 330755001903

[canadamom_of_2]

I want the shoes and they should have to ship them to me. I dont want a refund I want what I paid for. All I keep getting is the same email from them telling me to pay extra. Where is the buyer protection. I want what I paid for and thats that!