UPDATE: Now instead of just allowing you to test how reliable a response rate difference was between two packages, you can also calculate ROI and Net Revenue of those packages! Click on the 'Net Revenue' item on the top blue bar, enter in the costs and revenue associated with your two packages, and you will see the results in a pair of graphs in the main part of the app.
My one caution here is this: Just because a letter package was profitable, or had higher ROI, in this instance, it doesn't mean you should expect it to always be that way. After all, the package might only have been profitable because of the donations of under a handful of people.
This tool allows you to enter in the results of an A/B test that you've done on a letter package or email campaign. Just enter in how many letters or emails were sent and how many donations were received or how many emails were opened.
It will then show you a graph of the response rates or open rates and tell you whether the difference is statistically reliable enough that you should favour whichever condition led to a better result.
Let me know if you have any questions!