How to Offer a Free Trial and Recurring Payments Using Express Checkout.curl

CreateRecurringPaymentsProfile

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=CreateRecurringPaymentsProfile
&VERSION=94
&TOKEN=insert_token_value_here 
&PAYERID=payer_id_value   #Identifies the customer's account
&PROFILESTARTDATE=2012-09-12T00:00:00Z    #Billing date start, in UTC/GMT format
&DESC=PhotoStorage    #Profile description - same as billing agreement description
&BILLINGPERIOD=Month    #Period of time between billings
&BILLINGFREQUENCY=1    #Frequency of charges 
&AMT=1    #The amount the buyer will pay in a payment period
&TRIALBILLINGPERIOD=Month    #Period of time in one trial period
&TRIALBILLINGFREQUENCY=1    #Frequency of charges, if any, during the trial period
&TRIALTOTALBILLINGCYCLES=1    #Length of trial period
&TRIALAMT=0    #Payment amount (can be 0) during the trial period
&CURRENCYCODE=USD    #The currency, e.g. US dollars
&COUNTRYCODE=US    #The country code, e.g. US  
&MAXFAILEDPAYMENTS=3    #Maximum failed payments before suspension of the profile

Response
--------
PROFILEID=I%2d6D5UGCVX1234
&PROFILESTATUS=ActiveProfile
&ACK=Success
...

GetExpressCheckoutDetails

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=GetExpressCheckoutDetails
&VERSION=94
&TOKEN=insert_token_value_here

Response
--------
TOKEN=token_value
&BILLINGAGREEMENTACCEPTEDSTATUS=1
&ACK=Success
&PAYERID=3TXTXECKF1234
...

SetExpressCheckout

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=SetExpressCheckout
&VERSION=94
&L_BILLINGTYPE0=RecurringPayments    #The type of billing agreement
&L_BILLINGAGREEMENTDESCRIPTION0=PhotoStorage    #The description of the billing agreement
&cancelUrl=http://www.yourdomain.com/cancel.html    #For use if the consumer decides not to proceed with payment 
&returnUrl=http://www.yourdomain.com/success.html   #For use if the consumer proceeds with payment 

Response
--------
TOKEN=EC%2d2B984685J43051234
&ACK=Success
...