Because it sounds like you are going to be itemizing the order items (product, shipping, tax, discount) I think the XmlRpc InvoiceService methods will suit your needs best.
Of course, you can always use both the XmlRpc and Rest v1 and v2 APIs (Contacts and Tags APIs were released this morning).
Most accomplish this by wrapping their API Client and utilizing whichever API endpoint is best for their needs.