Call paypal using a POST request.

When the URL is too long (~4500 characters), Paypal is rejecting the call with :
TIMESTAMP=2013%2d07%2d01T06%3a45%3a59Z&CORRELATIONID=573b84f5c4852&ACK=Failure&L_ERRORCODE0=10001&L_SHORTMESSAGE0=Internal%20Error&L_LONGMESSAGE0=Timeout%20processing%20request

Using post instead of get is fixing the issue.