You might also try using XSL:FO. XSL:FO is a documented standard for describing page layout: http://www.w3.org/TR/xsl/#fo-section.
I've had success on two projects creating documents by creating an XML schema that defines the content of the "PDF". I then use the XSD tool (from Microsoft) to generate a class representing this document. I then map my data into that structure, serialize the populated class to XML, along with an XSL stylesheet that defines how that data should be mapped into FO, and pass it to an FO formatter. For formatters, I have use Alt-Soft's Xml2Pdf with success. There are a few others out there. There are some tools available to help create the XSL to FO stylesheet (i.e. stylusstudio and XmlSpy), but I recommend learning the FO constructs as the tools seem to produce bloated stylesheets. FO is comparable to HTML (where a P tag is a BLOCK tag in FO), but can be tricky. This nice thing about FO, is that some formatter support conversion to other formats, such as Word, HTML, etc.
Other options:
iTextSharp (C# port of iText). Just started reading about this. Open source and free. I don't think there is any "templating" supported with this, but I could be wrong about that.
SQL Server Reporting Services. Assuming your invoice data is in, or can be put in, a format that can be read by reporting services (SQL Server, Web Service, etc), define the layout in SSRS and then publish to reporting server. Use SSRS Web Services or query parameter execution to execute the report and have it output as PDF.