Review the original item:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns,
Bo was pretty sure that sticking the huge fruit under his shirt and
pretending that he was pregnant would not fool the cashiers.
You wanted to fix it this way:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns Bo was pretty sure that sticking
the huge fruit under his shirt and pretending that he was pregnant would
not fool the cashiers.
To delete the comma between buns and Bo would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. He had money only for hot dogs and buns is the first main clause. Bo was pretty sure is the second main clause.
The spot between buns and Bo needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.