"We need to make sure that we strike that right balance between driving value to the consumer and the merchant on small-ticket" purchases, Mr. Sheedy said.

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Shashi Verma, TfL's director of customer experience, said: ''Pay as you go Oyster cards do not expire and customers are able to return their cards to a Tube ticket office at any time for a refund of the remaining balance and card deposit.

BBC: Unused Oyster cards valued at ?55m

But that is something Selig wants desperately to avoid because he feels it would jeopardize the competitive balance of the sport, especially teams like the Rays and Cleveland Indians who are suffering from low ticket revenue but are currently atop their divisions.

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Clinton needs to find the right balance between selling her experiences and skills and keeping the excitement that her candidacy could bring to a Democratic ticket.

CNN: Hillary Clinton and the experience trap