QSYS2.HTTP_GET problem with the USPS APIs

[Sam_L <lennon_s_j@xxxxxxxxxxx>]

I am experimenting with QSYS2.HTTP_GET and the USPS APIs, Using Run SQL 
Scripts on PUB400.COM.
And I'm having a problem...

This example,taken from their web site, works in Postman, and also 
pasting into Firefox.

http://production.shippingapis.com/ShippingAPI.dll?API=Verify
&XML=
<AddressValidateRequest USERID="xxxxxxxxx">
<Revision>1</Revision>
<Address ID="0">
    <Address1>SUITE K</Address1>
    <Address2>29851 Aventura</Address2>
    <City/>
    <State>CA</State>
    <Zip5>92688</Zip5>
    <Zip4/>
</Address>
</AddressValidateRequest>

In Run SQL Scripts I thought code like below would work, but it always 
returns error 80040B19,
with the message "XML Syntax Error: Please check the XML request to see 
if it can be parsed."

values
QSYS2.HTTP_GET('http://production.shippingapis.com/ShippingAPI.dll' concat
url_encode('?API=Verify' concat
    '&XML='concat
    '<AddressValidateRequest USERID="xxxxxxxxx">' concat
        '<Revision>1</Revision>' concat
        '<Address ID="1">' concat
            '<Address1></Address1>' concat
            '<Address2>8 Wildwood Drive</Address2>' concat
            '<City>Old Lyme</City>' concat
            '<State>CT</State>' concat
            '<Zip5>06371</Zip5>' concat
            '<Zip4></Zip4>' concat
        '</Address>' concat
    '</AddressValidateRequest>'),
    '{
        "header":"Content-Type,text/xml",
        "header":"Accept,*",
        "header":"Content type,charset=UTF-8"
     }'
)
;

Can anyone suggest what I'm doing wrong?

The USERID is correct, just XXXed out here.
The XML validates.
I probably don't need the headers--they don't seem to make any difference.

USPS info, if necessary:

https://www.usps.com/business/web-tools-apis/general-api-developer-guide.htm#_Toc24631955

Client Protocols

HTTP/HTTPS protocols are used for exchange of data.  This is to facility 
passage through corporate firewalls and inclusion of multiple 
technologies for implementation. The simple example of this is a URL in 
a standard browser of the form:

http://production.shippingapis.com/ShippingAPI.dll?
API=APINAME&XML=<APINAMEREQUEST USERNAME=’your account’>
<tag>data</tag><tag1>data</tag1></APINAMEREQUEST>

https://www.usps.com/business/web-tools-apis/address-information-api.htm#_Toc39492052