Hmm...I am just taking a guess here but 5 ponies plus packing materials is going to be around 1lb 4 ounces, give or take. Which means they will have to send Priority and that should roughly cost around $6 and that is without a discount for printing it online. BUT....that same amount sent in a Flat Rate Box would cost around $17.45, again without the online discount.
I would probably send your seller a message and ask how she/he intends to ship? If it's stated in their listing that it's going flat rate your going to have a hard time backing out of the sale.