If you were saying the words aloud to someone would you pause slightly or not after the word "coffee"? You might, if you wanted to warn the listener that some surprise was on the way. "She was in the coffee shop, seeing if they also sold engine oil." But otherwise there is no need at all for a comma in your sentence.

Where you put commas is generally one of those matters of taste that nevertheless excite fierce disagreement amongst some native speakers. But @Edwin Ashworth rightly draws attention to the use of a comma in the very particular case that you are defining something or somebody. In that case leaving out the comma can cause confusion.

If you were saying the words aloud to someone would you pause slightly or not after the word "coffee"? You might, if you wanted to warn the listener that some surprise was on the way. "She was in the coffee shop, seeing if they also sold engine oil." But otherwise there is no need at all for a comma in your sentence.

Where you put commas is generally one of those matters of taste that nevertheless excites fierce disagreement amongst some native speakers. But @Edwin Ashworth rightly draws attention to the use of a comma in the very particular case that you are defining something or somebody. In that case leaving out the comma can cause confusion.