Hosting a C# Click Once desktop application online

I've had a C# desktop application developed for a customer that basically simplifies a Magento back end down to just the functionality she needs, as she found Magento too complicated. She uses an older mac, however, so running the tool in a VM proved to be too laggy.

I'm thus looking to host it in the cloud, but have been somewhat overwhelmed by the options available. From my research, Amazon EC2 with XenApp installed via the Amazon Marketplace seems to be a recommended choice. Would you agree? I've looked into AppHarbor as well, but unless I'm wrong I don't think they're able to host desktop apps.

The only potential constraints are that the application needs to make outbound connections over HTTP to the Magento back end and it also issues outbound SQL calls direct to the Magento database. In addition, it's only for one user so the cheaper the better.

Thanks!