This web service allows for the updating of line items on purchase orders.  Line item prices may be set and locked.

The Purchase Order Web Service is a SOAP web service that allows a Connect Supplier to manipulate a purchase order.  Currently, there is a single web method--UpdatePOLineItemUnitPrice--which can be used to mark a line item as being fulfilled and to set pricing.
 
Notes on method parameters:

• PayloadID is used to identify the purchase order to manipulate.You’ll need to get PayloadIDs from other interop services – the Purchase Order Web Service does not provide an ability to request them.
• The LockLineItem is an optional Boolean value.Line items cannot be unlocked, so once it is set to true for a line item, one can no longer set it back to false. If false is sent for that line item at a future date, a successful response will be sent back, but it will not be updated.Line items locked will be marked as fulfilled in the application.

With the UpdatePOLineItemUnitPrice web service, one can update multiple line items at once for the unit price and/or the LockLineItem setting.Use one LineItemUnitPrice element for each line item you would like to update.The LineNumber element indicates which line item should be updated.Line numbers are relative to the purchase order and begin at 1.

Framework:
All examples are written in C# utilizing the 3.5 .Net Framework.        

URL:
http://www.four51.com/services/PurchaseOrder.asmx?id=encryptedkey  
You’ll need to use a URL specific to your company.See the “Shared Key and URL” section for details.

Web Methods:
UpdatePOLineItemUnitPrice:Used to set line item pricing and mark a line item as fulfilled.

Shared Key and URL

The URL to send the SOAP requests along with the ability to generate a shared secret is not exposed in the Four51 Supplier interface.  In order to get the URL and secret, you’ll need to submit a case to Four51. 

Reference Material: 
Four51 Admin Web Service Implementation Guide