the currency is also set on the checkout page.
On the checkout page. look for the following code:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "USD";
and change it to:
$WA_PP_ECO_Set_params[0][$nextIndex] = "Currency";
$WA_PP_ECO_Set_params[1][$nextIndex] = "EUR";