One day Travelcard Z1-6 is also valid on National Rail, so it covers
most of your way already. So from £14.50 you pay for the Travelcard
(most of the amount) + rail ticket from boundary of Z6 to
Sawbridgeworth.

Also the travelcard ceases to be valid upon return to Sawbridgeworth whereas
a travelcard bought within the zones does not expire until the close of
service. This would offset any minor savings as it prevents you from say
shopping during the day, returning home and going back out in the evening on
the single ticket.