I was reading Robert Rossney's entry on "What's the most unsound program you've had to maintain?" found at: (What's the most unsound program you've had to maintain?) when I realized that I had inadvertently developed a near-identical application!
The app consists of an HTTPListener object that grabs incoming POST requests. Based on the information in the header, I pass the body of the request to SQL Server to perform the appropriate transaction.
The requests look like:

<InvoiceCreate Control="389>
  <Invoice>
    <CustomerNumber>5555</CustomerNumber>
    <Total>300.00</Total>
    <RushOrder>1</RushOrder>
  </Invoice>
</InvoiceCreate>

Once it's received by the HTTPListener object, I perform the required INSERT to the Invoice table using SQL Server's built-in XML handling functionality via a stored procedure:

  INSERT INTO Invoice (InvoiceNumber, CustomerNumber, Total, RushOrder)
  SELECT @NEW_INVOICE_NUMBER,  
         @XML.value('(InvoiceCreate/Invoice/CustomerNumber)[1]', 'varchar(10)'),
         @XML.value('(InvoiceCreate/Invoice/Total)[1]', 'varchar(10)'),
         @XML.value('(InvoiceCreate/Invoice/Total)[1]', 'varchar(10)')  

I then use another SELECT statement in the same stored procedure to return the value of the new Invoice Number that was inserted into the Invoices table:

SELECT @NEW_INVOICE_NUMBER FOR XML PATH 'InvoiceCreateAck'  

I then read the generated XML using a SQL data reader object in C# and use it as the response of the HTTPListener object.

My issue is, I'm noticing that Robert is indeed correct. All of my application logic exists inside the stored procedure, so I find myself having to do a lot of error-checking (i.e. validating the customer number and invoicenumber values) inside the stored procedure.

I'm still a midlevel developer, and as such, am looking to improve. Given the original post, and my current architecture, what could I have done differently to improve the application? Are there any patterns or best practices that I could refer to? What approach would you have taken? I'm open to any and all criticism, as I'd like to do my part to reduce the amount of "unsound programming" in the world.