We have recently moved away from the Pay per Study option as it was not providing value to our customers.
The monthly plan will provide access to our full suite of tools and can be unsubscribed to whenever you wish; therefore you're not locked into the any more than the number of months you need. This plan also gives access to the features that were locked out of the old pay-per-study option such as the URL redirect feature.
Also, the monthly plan provides access to a full suite of tools; which is the most effective way to use our software. This allows for iteration testing in order to check different versions of your design, AB testing, and as mentioned, access to all 5 tools rather than limiting you to just one.