Your TaxRate session variable is being set to the billing country from the form:
<%
if (cStr(cStr(Request.Form("billingcountry"))) <> "") then
Session("TaxRate") = "" & cStr(cStr(Request.Form("billingcountry"))) & ""
end if
%>
In the case of your error, it is getting set to "Germany", you cannot use a string in mathematical operation.