When someone logs in to PayPal on their computer PayPal sets a browser cookie identifying that person as a PayPal account holder. Every time that person returns to PayPal it will automatically display the login screen instead of the credit card checkout screen.
Since vendors using PayPal are almost always guaranteed to have logged in to their own PayPal account at some point in the past, vendors will always see the login screen instead of the credit card form.
PayPal does this, not DPD. There is nothing we can do to force a credit card checkout screen if the buyer's computer has the PayPal cookie set.
If you would like to see the non-PayPal account holder checkout for your product, you will need to manually delete the cookies stored in your browser.