I have troubles with this simple question: Cashiers do not understand my accent, so I want to find some answer that is easier to articulate.

When I answer " I already have one " - they never understand.

Is there any other option to say (apart of "No, thanks) that I have a bag (I brought it with me) that utilizes "better" vowels for non-native speaker?

British English UPD To those answering later: I want to improve my English pronunciation. The example in question is just one of the examples, not the whole issue, thus pantomimic clowneries could not be taken into account: NO, THANKS!