ShipStation has an XML API (PDF). Unfornately, because of poor PDF formatting, its impossible to properly copy-paste the XML validation schema. The following is the properly formated schema (as far as I can tell). Save it as a .xsd file and use it to validate your app’s output.
The email regular expression may change due to your language of choice. Originally, the PDF used:
([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+```
However, PHP didn’t like the \@
so I changed it to @
. Your mileage may vary.