This might be a weird suggestion, but you could try writing down what you want on a note and passing it to the worker. I don't know, that's probably a weird suggestion, but it might work. Have you ever seen or heard of the movie "Dear Frankie"? It is a very good movie which takes place in Scotland and the boy in it is deaf, so there is a scene where he goes into a fish and chips shop and just passes the woman a note for what he wants! I just had a weird feeling after watching that movie because it's like that boy is deaf and can't verbally communicate, but he was still able to go in and get his food and I should be able to too!
Or I'd say to just keep going in and trying. You're bound to get better at speaking clearly so that the woman hears you the first time. Before you walk into the shop make sure you have a script in your head (hell, even write it down if you really need to) about your exact order and what you want to say.