Both singular and plural solutions — sindīs, sindītīs — should be accepted for sentences like this, where the English doesn't clearly specify number.
Shouldn't "havon" be "havōn" as well?
It seems like the conjunctive lengthening strategy only applies to the ultimate item in a list. Not sure, though. Maybe it would be different if the intended translation was 'buy meat AND bread AND eggs'?
Well, I got a message that my solution with both parklon and dröma lengthened was accepted, so either they made a mistake, or both apply?