In your Seller Admin > Edit Account Preferences, you have entered a Common Notification URL of "https://mail.google.com/mail/u/1/#inbox", which is causing the notification errors you noted. Putting that URL there would serve no useful purpose anyway, so you should simply delete it.
On that same screen, you have entered a Common Thank-you Page URL pointing to a page on your site. This means buyers will be redirected to that page after checkout, instead of our standard thank-you/download page, so they would need to wait for product-specific thank-you emails providing a link to their download page for each file.
If you want to let your buyers download immediately following checkout, you should delete your Common Thank-you Page URL.
However, if you'd prefer to direct buyers back to your site after checkout and make them wait for thank-you emails providing access to their downloads, in this case you can keep the Common Thank-you Page URL but should probably just delete your Common Thank-you Email template to disable that message (this will not affect product-specific emails).