This kind of question has come up several times before before. I think different peple have different opinions. For me, "lo and behold" is parenthetical, so logically should set off by a pair of commas " ..., lo and behold, ...". However, in sentences with this structure, such an approach can create other problems.
I agree, the sentence starts with an action verb (opened) and is followed by a statement rather than a second action. It might instead read “I opened the box. Lo and behold, there was a ring inside.”