Might be worth contacting an agent or two who could conceivably work on your behalf. You give up 15 percent, but you may also gain access to foreign rights sales and film options, depending on how canny and ambitious your agent is. Given that you already have an offer, it may be easier to find an agent. In my own (dated) experience, my agent made foreign rights and film sales that added up to quite a bit more than the US publisher's advance.
My novels, Valentine's Cafe, Thereafter, Darkest Desire: The Wolf's Own Tale, and Mermaid in Vegas are available at Amazon
. Blog: Books and Boats.