Originally Posted by Kamakiri
Please do NOT say things like "arigatou gozaimashita" to store clerks; it's bizarre and unnatural. Don't even bother with "arigatou." A simple "doumou" will be precisely what you should say and what they will expect.
There are regional differences on this point. In Tokyo, it is indeed uncommon for customers to thank store clerks. In Kansai, though, it is considered polite and normal to say arigatou
, or ookini