We have deployed this feature on busy merchants and it is working very well

If you invoice clients abroad, and are paid in Visa/MasterCard etc etc you can invoice using our invoicing feature

http://blog.pesapal.com/how-to-invoice-your-clients/

Lots of tour operators are using this and the largest payment you can process is USD50,000

We plan to do a developer session about IPN and our Invoicing API