suggestions:

1st on sign-up.. when entering a password, you can show the information about secure password before submitting
2nd on adding a notification the label "free of charge" should be changing right after ticking the sms-checkbox
3rd give me an (optional) input field to label my "notification" like "donations for the blog" or "bike-shop" so that I can see on first glace whats going on.

but apart of these really small things your service is very well done!

how many blockexplorer-servers are there redundant - just in case?