You need to double the final consonant before adding the suffix for "shop + -er."
In this case, "shop" ends with a single vowel followed by a single consonant, and the suffix "-er" is added, so you double the final consonant to spell "shopper."
The other examples do not require doubling the final consonant:
- "limit + -ed" becomes "limited" (no doubling)
- "fear + -ing" becomes "fearing" (no doubling)
- "cost + -ly" becomes "costly" (no doubling)