edtFTPnet is under the Lesser GNU Public License (LGPL), and hence can be freely used in commercial or non-commercial products.
It should be noted, however, that under the LGPL
* edtFTPnet must be used as a library (i.e. a DLL), and not directly incorporated into your codebase, and
* if you make any modifications to edtFTPnet and wish to distribute an application that uses these modifications, you must make the modified library available under the LGPL.
There are other implications of the LGPL - read the license for more details. Because of your requirements (e.g. modifications you wish to make) you may wish to purchase a licence unencumbered by the LGPL.
See
http://www.enterprisedt.com/products/ed ... chase.html for more details and pricing (very reasonable).