How to Accept a Basic Digital Goods Payment Using Express Checkout.curl

DoExpressCheckoutPayment

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=merchant_user_name
&PWD=merchant_password
&SIGNATURE=merchant_signature
&METHOD=DoExpressCheckoutPayment
&VERSION=89
&TOKEN=token_value
&PAYERID=3TXTXECKF1234   #From the GetExpressCheckoutDetails response; identifies the customer's account
&PAYMENTREQUEST_0_PAYMENTACTION=SALE  #Indicates that a payment will be processed
&PAYMENTREQUEST_0_AMT=21   #The amount of payment
&PAYMENTREQUEST_0_CURRENCYCODE=USD   #The currency, e.g. US dollars

Response
--------
TOKEN=token_value
&ACK=Success
&PAYMENTINFO_0_TRANSACTIONID=3CA20865912341234
...

GetExpressCheckoutDetails

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=merchant_user_name
&PWD=merchant_password
&SIGNATURE=merchant_signature
&METHOD=GetExpressCheckoutDetails
&VERSION=89
&TOKEN=token_value

Response
--------
TOKEN=token_value
&ACK=Success
&PAYERID=3TXTXECKF1234   #Identifies the customer's account
&L_PAYMENTREQUEST_0_ITEMCATEGORY0=Digital
...

SetExpressCheckout

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=merchant_user_name
&PWD=merchant_password
&SIGNATURE=merchant_signature
&METHOD=SetExpressCheckout
&VERSION=89
&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
&PAYMENTREQUEST_0_CURRENCYCODE=USD    #The currency, e.g. US dollars
&PAYMENTREQUEST_0_PAYMENTACTION=SALE    #Payment for a sale  
&PAYMENTREQUEST_0_AMT=21    #The amount authorized
&PAYMENTREQUEST_0_ITEMAMT=21
&L_PAYMENTREQUEST_0_ITEMCATEGORY0=Digital    #The item category must be set to Digital 
&L_PAYMENTREQUEST_0_NAME0=my_movie
&L_PAYMENTREQUEST_0_QTY0=1
&L_PAYMENTREQUEST_0_AMT0=21

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