I have been designing some of the payment and plan upgrade screens for our upcoming web application, and while doing a little bit of research I discovered something interesting.
Basecamp (above) does not charge incrementally. If you upgrade your plan a day after your billing cycle you’ll have an entire month at the previous rate.
FreshBooks (above) on the other hand charges you incrementally based on changes you make. So if you add extra staff to your plan for example, you’ll have to pay.
Both ways are fine, but it’s just an interesting thing to consider when you build your own web app.