OK, wait, I see the problem now.
Line 377 of the ordersummary.asp page, you are setting the billingcountry form field to hold the country name that is stored in the billingcountryhidden form element:
<input name="billingcountry" type="hidden" id="billingcountry" value="<%=cStr(Request.Form("billingcountryhidden"))%>">
change that to:
<input name="billingcountry" type="hidden" id="billingcountry" value="<%=cStr(Request.Form("billingcountry"))%>">
so that it maintains the taxrate value