http://getdpd.com is the best option I've found. The interface makes sense, the pricing is super fair, and they allow you to send unlimited updates for free to your users (most other services charge more or, even worse, per update!).
From their website (http://getdpd.com/plans-and-pricing/): "What types of payment do you accept?
We accept Visa, Mastercard, and American Express. At this time we only accept payments online so we will not be able to accept a P.O., invoice you, or take an order over the phone."