Your judgment is right in that you need to write a SOAP web service exposing a single operation called notifications. Your client, viz. Dupli Envelope & Graphics (Matt Industries Inc.), has its web service situated at [login to view URL] and it is implemented using SalesForce and uses SOAP as communication protocol. Any SOAP web service supports functionality to INTEROPERATE with other SOAP web services (whether internal or external) as part of SOAP specification. So, as they already have the functionality to post file in place (as part of their web service), they are asking you to create a SOAP web service situated at a particular endpoint. Thus, you client can "plug-in" your SOAP web service into their own so that INTEROP will be possible.
In [login to view URL], they have supplied the Data Dictionary XSD and other information that PRECISELY defines how the SOAP web service needs to be implemented. Don't worry! I have FULL CONFIDENCE of implementing this SOAP web service in either VB.NET or PHP.
Although new to Freelancer.com, I have extensive experience in web services, SOAP, XML technologies, PHP and .NET.
Thanks,
Tushar