...

ptbletter/ptbletter.cls

@@ -122,6 +122,8 @@
 \defineshorthand{"'}{\closeautoquote}
 \AtEndOfClass{\shorthandon{"}}
 
+\RequirePackage{amsmath,siunitx-PTB}
+
 \@setplength{toaddrhpos}{72bp}
 \@setplength{toaddrvpos}{124bp}
 %\@setplength{toaddrwidth}{85mm}