Place of 了 particle for the past tense?
Both are correct for past tense. They have the same meaning, with subtle difference on the point of emphasis.

"我去了商店" lays emphasis on the action "went to the shop", while "我去商店了" lays emphasis on the destination of the action, i.e. "the shop".

So it'll be more natural to use "我去了商店" as the answer to the question asking "what did you do" (你做什么去了), and use "我去商店了" as the answer to the question asking "where did you go" (你去哪儿了). (Note that they are just more targeted and straightforward to the answer, using another one as the answer is fine too.)