With PayPal.me you can pre-select a payment with a username and amount, but it is possible to add other parameters?
PayPal.Me/username/amount/note/
PayPal.Me/username/100/ordernumber123456
It's not possible with PayPal.me
For that type of functionality, instead log into http://www.paypal.com and check if you have the option to create a "Sell on Social" button at http://www.paypal.com/buttons
If you do not, a second alternative is to create a "Buy Now" button, leave the amount blank, do not change anything in the box of "Customize this button", and in Step 2 uncheck the option to save the button at PayPal. Then generate the code and switch to the "E-mail" tab.