Is this possible?

I went into a newsagent last night and tried to put £1 on my Oyster
pre-pay.

He didn't seem to understand and all he wanted to do was sell me a bus
pass. Luckily I managed to stop him doing it before he tried to
charge me!

I suspect it is possible to top-up pre-pay at newsagents, and I just
want to know how to phrase the question so that the newsagent will
understand.

TIA,
Steph