Hi Beckie,
It sounds like this may be in different issue than the one in your other thread. Could you look in your web server logs and see if there are any hits to your paypal-process.aspx page? If you aren't getting any hits, then it is a configuration problem. Also, make sure your Application URL is set correctly in your Configuration area in the admin.
If you are continuing to have problems, could you email support@dotnetinvoice.com so that we can get into your details and respond more quickly? If there is helpful information for everyone, we'll post it to the forums once we get it working.
---
Jeremy
DotNetInvoice Support