Jeremiah,
We don't require a written note from the other ISP. We do however require
the customer to get the other ISP to send us E-mail saying it is OK. We
won't route the prefixes unless we get this. I trust that e-mail won't be
forged. If the customer does manage to forge it and I get a subsequent
complaint from the ISP, who then asserts they never gave permission, I can
always disable the routes and deal with the dishonest customer.
You are doing just about the right thing. I think getting written permission
is a bit too officious.
Walt
Los Nettos