Cool idea! It sounds like you're offering it for free, with the option for a donation. How are you going to deal with the influx of what I imagine will be hundreds of free requests? Won't that get pretty expensive fast?
This seems like a clever way to collect valid mailing addresses. People are also likely to include personal information in their praise messages, which could be valuable data.
Their Terms and Conditions and Privacy Policy state they reserve the right to share collected information with service providers, business partners, and affiliates. They can use your data for "other purposes" including "data analysis" and "identifying trends." They can share your information with "business partners to offer you certain products, services or promotions."
*ianal
A USPS Forever Stamp is $0.73. Unless yall are rolling in VC funds or a lot of extra cash, a few hundred or even thousand orders is going to nuke the entire idea.
Are you still going to harvest and use all of the collected data of people who never got anything mailed?