It's not necessarily like coconutos's answer. In this case, the difference is present and past tence is not so important and we almost automatically separate them. Actually we Japanese say "ありがとうございます" to also past actions and some preffer it, because past tense may sound like appreciation ends just when they say it.

The difference is present and past tense. Everything in japanese that ends in masu is present and mashita is past. Example: A: Suwatte kudasai (please sit down) B:Arigato gozaimasu (thank you) vs. A:Tabemono dou deshita ka? (how was the food?) B: Oishikatta! Arigatou gozaimashita! (It was delicous! Thanks for making it.) Hope that helps.