Hi there, i've got a bit trouble with oyster card. the thing is that
yesterday i got in tube at king's cross station about 5pm, i'd topped
up 5 pounds before i got in. And then when i got off at ealing
broadway, it said the i had to pay extra 4 pounds for my journey(from
king's cross to ealing). i've got totally confused. they gave me a
penalty paper which says i have to pay it off in 21 days
otherwise...but i am pretty sure that i did top up successfully as i
also checked my banking statement.

What can i do now? call the service line or what else? i didn't oeder
a receipt when i finisded topping up at tube station unfortunately;s