Because E-junkie is a centrally-managed system shared in common among all merchants subscribed to our service, rather than software that gets installed on your own server, we would manage your secure checkout page for payment methods that don't use their own checkout site -- e.g. for PayPal Pro, Express Checkouts using a buyer's PayPal account would use PayPal's checkout site, whereas Direct Payments from a buyer's card would use our secure checkout page hosted on https://www.fatfreecartpro.com/...
Since this checkout page is shared in common among all our merchants, it cannot be modified for any one merchant alone. You can at least upload a custom logo/header image in your Seller Admin > Edit Profile, which would be displayed at the top of this checkout page.
To have checkouts transpire on your own domain, you would need to install a copy of "licensed" e-commerce software on your server along with an SSL certificate for your domain, rather than using a "hosted" e-commerce service such as E-junkie.