Security warning when returning from PayPal

Hi there,
I am wanting to use the Easy Digital Downloads plugin on a site I am building (currently using plugin version 1.3.4.3 on WordPress 3.4.2). I have noticed that, when in test/sandbox mode, after you complete the payment through PayPal and you click to be redirected back to the store to download your purchased products, a warning appears saying something along the lines of “Caution: information you are sending could now be seen by a third party” (longer than this but that’s the gist). If you click cancel, the payment emails go through and you can download the product, but the official purchase receipt doesn’t get sent and the sale doesn’t appear in the back end records. If you click okay, the purchase receipt is sent and the sale appears in the records. This seems a bit of an odd warning to be getting through what should be a secure process. Can you point me in the right direction as to why this might be occurring and how to fix it? Any help would be greatly appreciated!

http://wordpress.org/extend/plugins/easy-digital-downloads/