I'm currently working on a project which will use a j2me application on phone which will make a payment from a paypal users account, then send the token to another server which will verify the transaction, then carried out a task on the server.

Currently i'm looking into using the web services API as it should be possible to use SOAP based system in order to process the transaction via the Direct payment api, However i would like the ability for the user to use his/her paypal account to bill the charge.
As for security, is there a required certificate which needs to be on the phone in order to allow the user to do the transaction securely? and would this need to be preloaded onto the phone, or could it be pushed on when the transaction is completed?

If anyone has any experience using paypal on a j2me application, any advice would be appreciated.