When anyone say me to go and buy some product.. i get anxiety and think " how i speak in front of shopkeeper" and i only focus on that product name ..and then i get vocal blockage and i feel very shameless and cowerd. what can i do?

It is not feasible to provide an opinion without additional history, physical examination may be some tests. It would be prudent to see your doctor, preferably a psychiatrist. Such issues could be addressed through cognitive behavioral therapy. You should not delay seeking care as the condition is treatable. Wish you good health!

Ask or read up on that product so you are familiar with it and it's uses .Now when you go to buy it you will feel more comfortable and knowledgeable and less anxious when you speak to the shopkeeper. You will now feel more confident in yourself and not inclined to seek therapy.