new boxes depending on size are $10- $25 , if they sent them in recycled boxes, not so much.
Often on auction sites, people add extra to everything to cover back the expense of their auction, ie listing fees, auction selling fees, paypal fees, etc.
They arent supposed to, but they do.
eBay for example charges 12% on most auctions I run for eggs, then Paypal gets theirs, so by auction formate, it can get into the sellers pocket quick.
However, yes $105 for those was a touch high, especially if the quality was low on the birds.
$75 usually covers the box and shipping reguardless of how many birds are in it, or how far it has to go, unless it's huge birds like peafowl or something.