So when I go to Subway I always get either a turkey on Italian or a ham on Italian, and I load it up with 10 veggies: lettuce, tomatoes, spinach, cucumbers, green peppers, onions, olives, pickles, banana peppers, jalapenos. I always run into the problem of the sandwich artist trying to close and wrap my sandwich before I've added all of the veggies. I've found that if I say I want more than 2 veggies at once (for example, "lettuce, tomatoes, onions") they inevitably put the first two on the sandwich then look at me and have to ask what the third one is that I said, and then start to wrap up the sandwich. On the other hand, if I only say one at a time (for example, "lettuce") they put on the one thing I said and start wrapping up the sandwich.
I've realized that this isn't a problem when the sandwich artist is an American person because the intonation (rising intonation until the last item in the list, then falling intonation to signal the end) tells them that I'm going to be continuing my list of veggies. It's only the foreign (i.e., 99% of them) artists that do the premature wrapping.
If I say something like "I'm going to get 10 veggies, starting with lettuce,..." they don't understand me, because they basically only respond to the names of vegetables.