I'm sorry you ran into an issue when using the plugin. Actually the Homepage Cache doesn't do anything to prompt users to upgrade to a newer browser, but your site template might actually be doing that (let's say with a Yootheme or RocketTheme template) since I've run into that issue myself on the orware site (which uses a Yootheme template).
It hasn't happened often, but I believe if someone visits your site at exactly the right moment (when the cache is being recreated) with an older browser (which would trigger the "upgrade your browser" message in those types of templates) then that particular page gets cached for everyone to see. Typically this feature is something you can turn off within your site template so it doesn't occur again, which may be something to try and might be helpful in your case.
The plugin itself is fairly simple and mainly provides the features I mention in the description above (cache the homepage specifically and nothing else, unless you specify a few additional pages that should be cached) so what you're mentioning is not being generated by the plugin itself (the plugin is just caching that particular response being returned by your template when a user with an older browser visits the page right at the moment when the old cache has become stale and a new one has to be recreated, which is then unfortunately displayed to subsequent users).
I've been meaning to take a look at this and see if I can go ahead and add the ability for the cache to be created on a per-browser basis (so Chrome browsers will have a separate cache from Internet Explorer browsers, etc.) and I believe that will help to resolve this type of issue from affecting other site visitors in the future (let's say in this case upgrade page was triggered by someone using Internet Explorer 7.0, but IE 8.0 would be OK...using this approach only the IE 7.0 folks would be affected and the IE 8.0 folks would have a separate, normal looking page returned).
If you ever have any issues feel free to send a direct email to firstname.lastname@example.org and I'm always happy to help sort through bugs like this and figure out a solution!
You can make tests with paypal sandbox and it logs the ipn activity! So you can use your component as an administrator to see how many payments you have received.
Thank you so much for the encouraging review. I really appreciate it.
Based on your description though, I believe you meant to leave this review for the Nice PayPal Downloads extension and not the Nice PayPal Button plugin. As the Nice PayPal Downloads works with IPN and the Nice PayPal Button does not.
In any event, I simply wanted to say thank you and to clarify for the sake of other readers.
~ All the best