How do you say 'umm' implying thinking or hesitation?

I thought it was 'eh'. But when a clerk is speaking to me in Spanish and I say 'eh' because I am trying to decide on something, they immediately switch to English. For example,

- Necesitas una bolsa?

- E.. (looking at the bag in my hand) no, porque ya tengo una.

- That will be $13.98.

- ??? Está bien si pago con tarjeta?

Is there something I should be saying instead, like 'a ver...'?