I really want a coffee.
Why not just ask the clerk to get a coffee for you?
I was too shy to ask the clerk for one coffee.
Sometimes we need help and clerks in a store are a good source of help. If you ask a clerk for help be sure to be kind about it. This way you'll get the best help and assistance possible.
If they were good at their job, they would know what I want without me even needing to ask them.
Clerks are not mind readers. Everyone has different needs so you should just tell them what you want. If you have anxiety you can go to therapy.