When living in Japan many years ago, at a donut store I would point and name the ones I wanted. But when I was done, I didn't know what to say. (so I would mumble something and gesture with my hands). I later asked my boss' wife about this, and she suggested 'sore dake ii desu' to show that I was finished. Is that a good closing statement?
The expression you need is 以上で (or 以上です). See: How to actually order fast food?
それだけでいいです (or それだけで) is also fine, but this contains だけ ("only/just"), so saying this after buying a dozen donuts and a drink would sound a bit funny.
(それだけいいです (without で) doesn't make sense in this context. Perhaps you remembered it incorrectly?)