WP Travel plugin provides the feature of adding a login/register option before making a booking of the trip. You can enable/disable the Require Login option by going to Admin Panel > WP Travel > Settings > Account
If you enable the Require Login option, the login/register section will be appeared before making a booking the trip.
If the login/register option is disabled(in case of partial payment) then the customer will be unable to make the remaining payment through user dashboard page as the payment made by the customer cannot be tracked. So, customer has to make the remaining payment manually.
Also, if the login/register option is enabled(in case of partial payment) then customer can make the remaining payment through user dashboard page easily.
If you have any confusion then please post them in our official support forum.