the long error is:
<LongMessage
xsi:type="xs:string">The transaction currency specified must be the same as previously
specified.</LongMessage>
if you change this code on the checkout page:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "USD";
you must also change this code on the pp_confirm.php page:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "Currency";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "USD";
to match.