Suit or shirt?
Do not be preoccupied with any assumptions like only suits are best considered for business purposes instead a crisp and neat shirt at times does much better than a hundred dollar suit!

Courtesy:
Being a businessman it is advisable to be thankful and courteous to those in the shop who assisted you in selection the best for you. So do not hesitate to thank them for their coordination. This will improve your chances of being served even better the next time you visit that shop.
If you follow the above given suggestions then you can assure yourself to be relived of the pain of shopping.