You've determined the ePublisher matches your manuscript. Look deeper, at the finer points of contracts, sales, and networking. If not, ask for a copy. It could be the ePublisher doesn't use a contract, in which case think carefully before proceeding. With or without a contract, keep a paper trail at all times.
Royalty rate is only one thing to consider when evaluating a contract.
Good ePublishers actively promote their site, titles, and authors with contests, newsletters, or other interaction to encourage repeat visitors. They also should keep their web site high on the search engines. Use a couple search engines and see where your ePublisher turns up on the list. Check if the ePublisher offer formats compatible with handheld devices, such as the Palm Pilot, or formats compatible with dedicated handheld eBook reader devices, such as RocketBook or SoftBook. Ask other writers if they've heard of the ePublisher or have had any experience with them.