I'm planning on buying a travelcard, which will give me travel to and from London and travel in zones 1-6. I need to get out to zone 9. Is it possible to buy a fare from Moor Park (the last station in zone 6) to Amersham and use my travelcard for the rest? If so, can I buy the fare when I reach Chesham, or do I need to get out at Moor park and get the next tube?