Forum Moderators: buckworks
(1) It would be best to select "encryption" for your button, otherwise it may be possible for your customer to "edit" the price and yet proceed.
(2) At the "Further Options", just underneath there is a return page for unsuccessful transaction (i.e. the purchaser does not wish to proceed). Make sure there is a link there, otherwise PayPal sends the purchaser to your download page by default.
Hope this helps.